Why do Baptists even baptize in water at all considering their "spirit baptism" views?

In addition to David Stratton's excellent answer as a long time Southern Baptist there are a couple of things I wish to add.

Not only do Baptists believe in Baptism, but it must also be immersion Baptism, and there are several reasons for that.

1. Jesus was immersed in the Jordan River when he was Baptized by John the Baptist.

Matthew 3:15 and 16 KJV And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him. And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:

Since Jesus was immersed it seems to follow that we too should also be immersed.

1. After Jesus came up out of the water the Spirit of God descended on him.

This was a sign that God was now pleased with Jesus for having been Baptized, as John said he should be baptized by Jesus; and Jesus answered we must fulfill all righteousness.

A Baptist Minister once explained to me the ritual of Baptism, as signifying three things.

1. Going down into the water, signifies our old sinful self being buried with Jesus.

2. Coming out of the water, signifies our becoming a new person acceptable to God, and his pride in our regeneration.

3. That we are now clean by having been washed in the blood of Jesus, and baptized with the Holy Spirit and fire.

Luke 3:16 KJV John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire:

So our idea of Baptism is that it is a way of announcing to the World that we are a new person (saved by the free Grace of God, and not by anything we have done).

This is in a way our answer to Jesus when he said:

Matthew 10:32 and 33 KJV Whosoever therefore shall confess me before men, him will I confess also before my Father which is in heaven. But whosoever shall deny me before men, him will I also deny before my Father which is in heaven.

So even though we do not believe that Baptism is necessary for Salvation, it is a very important part of our binding with Jesus, and a way of expressing our unity with Christ.

When the question is asked "Why was Jesus baptized?" acknowledging that He had no sin to repent of or be saved from, and when his answer to John the Baptist "Permit it to be so to fulfill all righteousness." is considered, it is practical to understand baptism as a symbolic act of identification. Jesus was identifying himself with sinful humanity (especially, in John's baptism) with the sinful nation of Israel, and by extension all who would believe, so that He could be our kinsman-redeemer.

Baptists hold that full immersion in the waters of baptism symbolically demonstrate their identification with the death (going under the water), burial (being held under for a short moment), and resurrection (coming up out of the water) of the Lord Jesus Christ; identifying with Him in like manner as He has identified with us. For Baptists, this is not salvatory but it is the first and primary step of obedience to the Lord in the new life. Like the Lord's Supper it is a public proclamation, albeit only a one time proclamation as opposed to the Supper which is to be repeated as a continual remembrance until He comes again.

Baptists practice baptism because it is something taught in Scripture.

Your question hits exactly on the distinction between those denominations that believe that baptism is necessary for salvation, and those that don't. Baptists don't see it as necessary for salvation. Instead, it's seen as an outward expression of obedience.

The analogy is graduation. getting your diploma doesn't impart knowledge onto you. Getting an education - sitting through class, studying, etc. imparts knowledge. Yet people participate in the graduation ceremony. Similarly, Baptists, and other groups that believe that baptism is not a necessary component of salvation practice it as a celebration, or an outward expression of the fact that they are already saved, and have already been born again of the spirit.

Backing reference: Baptists: Believer’s Baptism

Excerpt:

Baptists believe that the Bible teaches that baptism is important but not necessary for salvation. For example, the thief on the cross (Luke 23:39-43), Saul on the Damascus road (Acts 9:1-18) and the people gathered in Cornelius’ house (Acts 10:24-48) all experienced salvation without the necessity of baptism. In his sermon at Pentecost, Peter urged those who had repented and believed in Christ to be baptized, not that baptism was necessary for salvation but as a testimony that they had been saved (Acts 2:1-41).

Thus, baptism is symbolic and not sacramental. Baptists believe that the Bible teaches that baptism symbolizes that a person has been saved and is not a means of salvation. Baptism is not a means of channeling saving grace but rather is a way of testifying that saving grace has been experienced. It does not wash away sin but symbolizes the forgiveness of sin through faith in Christ.

While baptism is not essential for salvation, it is a very important requirement for obedience to the Lord. Christ commanded his disciples to baptize (Matthew 28:19) and therefore baptism is a form of obedience to Jesus as Lord. Baptism is one way that a person declares, “Jesus is Lord.”