Why do Catholics ask for things from an omniscient and omnibenevolent God?

You should at least be consistent If he is omniscient and still wants prayer it is NOT because HE doesn't know certain things. You are merely doubting God's omniscience in the matter of prayer. Same with omnibenevolence, are you thinking that in this case God is being evil or asking for something just to be gratuitous about things ?

Thunderforge's answer is excellent. I add only a few points:

1) We pray so that we may dispel anxiety and enjoy peace. Philippians 4:6-7 says:

Do not be anxious about anything, but in every situation, by prayer and petition, with thanksgiving, present your requests to God. And the peace of God, which transcends all understanding, will guard your hearts and your minds in Christ Jesus.

2) We pray because we are strongly urged by the Apostle Paul to do so in Ephesians 6:18-20, which says:

And pray in the Spirit on all occasions with all kinds of prayers and requests. With this in mind, be alert and always keep on praying for all the Lord’s people. Pray also for me, that whenever I speak, words may be given me so that I will fearlessly make known the mystery of the gospel, for which I am an ambassador in chains. Pray that I may declare it fearlessly, as I should.

Note that by praying for other people, we show them love, fulfilling the great commandment.

3) We pray in imitation of our savior, who in scripture was shown praying about three dozen times:

See http://jesusalive.cc/ques204.htm

4) And most importantly, we pray so that when our specific, intentional, God-directed prayers are answered in ways we cannot explain, our faith will be strengthened and our witness to others enlarged. My faith taught me to pray before I had ever seen any prayers answered, and my faith grows each time a prayer is answered, and sometimes when it is not. "No" is a valid answer.

One nice thing about the Catholic Church is that, because they have 2000 years of history to draw from, they have written a great deal about most every question of faith and have codified it as well. The Catechism of the Catholic Church (CCC) #2629-2633 discusses a prayer of petition (which is the technical term for a prayer where you ask God for something), and the reasons that it is done. In all the passages below, the bolding for emphasis is mine, any italics were in the original.

To express our reliance on God

By asking things of God, we acknowledge that we are not able to do everything ourselves and are reliant on him. When we ask God for things outside our control, we begin a process of turning away from our sin (I imagine that pride would be a notable sin in this case) and turning instead to him. In other words, God knows our needs, but we are acknowledging that we cannot provide for those needs ourselves and that instead we must rely on him to fulfill those needs.

2629: The vocabulary of supplication in the New Testament is rich in shades of meaning: ask, beseech, plead, invoke, entreat, cry out, even "struggle in prayer."¹⁰² Its most usual form, because the most spontaneous, is petition: by prayer of petition we express awareness of our relationship with God. We are creatures who are not our own beginning, not the masters of adversity, not our own last end. We are sinners who as Christians know that we have turned away from our Father. Our petition is already a turning back to him.

_{102 Cf. Rom 15:30; Col 4:12.}

To give us hope in our lamentation, and to receive help in our other prayers

By asking God for things, we are provided hope from our struggles. Additionally, the Holy Spirit helps us to pray both in petition and other types of prayers.

2630: The New Testament contains scarcely any prayers of lamentation, so frequent in the Old Testament. In the risen Christ the Church's petition is buoyed by hope, even if we still wait in a state of expectation and must be converted anew every day. Christian petition, what St. Paul calls {"groaning," arises from another depth, that of creation "in labor pains" and that of ourselves "as we wait for the redemption of our bodies. For in this hope we were saved."¹⁰³ In the end, however, "with sighs too deep for words" the Holy Spirit "helps us in our weakness; for we do not know how to pray as we ought, but the Spirit himself intercedes for us with sighs too deep for words."¹⁰⁴

_{103 Rom 8:22-24.
104 Rom 8:26.}

In the case of asking for forgiveness, to be able to make other types of prayers

Other types of prayer, as well as the Eucharistic liturgy, require asking for forgiveness before they can be performed.

2631: The first movement of the prayer of petition is asking forgiveness, like the tax collector in the parable: "God, be merciful to me a sinner!"¹⁰⁵ It is a prerequisite for righteous and pure prayer. A trusting humility brings us back into the light of communion between the Father and his Son Jesus Christ and with one another, so that "we receive from him whatever we ask."¹⁰⁶ Asking forgiveness is the prerequisite for both the Eucharistic liturgy and personal prayer.

_{105 Lk 18:13.
106 1 Jn 3:22; cf. 1:7-2:2.}

To express our desire and search for the Kingdom of God

We are desiring and searching ourselves for the Kingdom of God, and our petitions should be for things that will help in that.

2632: Christian petition is centered on the desire and search for the Kingdom to come, in keeping with the teaching of Christ.¹⁰⁷ There is a hierarchy in these petitions: we pray first for the Kingdom, then for what is necessary to welcome it and cooperate with its coming. This collaboration with the mission of Christ and the Holy Spirit, which is now that of the Church, is the object of the prayer of the apostolic community.¹⁰⁸ It is the prayer of Paul, the apostle par excellence, which reveals to us how the divine solicitude for all the churches ought to inspire Christian prayer.¹⁰⁹ By prayer every baptized person works for the coming of the Kingdom.

_{107 Cf. Mt 6:10,33; Lk 11:2,13.
108 Cf. Acts 6:6; 13:3.
109 Cf. Rom 10:1; Eph 1:16-23; Phil 1:9-11; Col 1:3-6; 4:3-4,12.}

I'd like to highlight Matthew 6:33 (from footnote 107), which says "But seek first the kingdom [of God] and his righteousness, and all these things will be given you besides."

This passage also makes it clear that the things that we should ask for are those that are necessary to bring the Kingdom of God.

To make us prepared for what God is intending to give

In the section of filial piety in prayer, CCC #2737 talks about why we don't immediately receive what we ask for and quotes St. Augustine (b. 334–d. 440) in a passage that is relevant for this as well:

God wills that our desire should be exercised in prayer, that we may be able to receive what He is prepared to give.

Summary

It's clear from the Catechism that the reasons for prayer are less about helping God out (because God certainly has the capability and desire to give us what we he knows we need), but more about aligning ourselves to God's desires. Prayer makes us able to understand that we need God to fulfill our needs and prepares us to receive them, while also providing us hope through Christ. In the case of asking for forgiveness, it also a prerequisite for other types of prayer.