From a Catholic viewpoint, why will there be a particular judgement and a general judgment?

The Short Answer, our standing before God at the time of our death is determined concerning salvation and damnation. If sanctification is not completed do to sins committed after baptism and atonement yet to be made in and through the atoning sacrifice of the Cross, that will be done before entering into the Presence of the our Father in heaven.

The General Judgment in not a separate judgment for those who have already passed, but will be a particular judgment for those who are still alive on the last day. At this time, all deeds good and bad will be made known to all so that the consequences of the actions of sin and righteousness can be made known to all.

To those whose sins are not washed by the sanctifying Grace of our Lord Jesus Christ by an obedience of Faith and Baptism, Or sanctified by some mysterious way known only to God, those persons sins will be judged for damnation and not to the Glory of God.

To those whose sins are washed by Baptism and through the Ministry of Reconciliation, which is an obedience of Faith, those sins now forgiven, will be presented to all present for the Glory of God and his Mercy.

Like Peter, when he denied Christ three times to his shame, had a full heart filled repentance before God to his Glory by showing Gods Mercy even after knowing and denying his only Son.

There is also the cascade effect that sin causes in the world and this effect will be made manifest.

Example: You are given an opportunity to defend the Catholic Faith to a person who asks you to defend it. You choose not to do so because you are perhaps intimidated by that persons own faith practice, which you know, is false. By not taking the opportunity to proclaim the Gospel as given by Christ, that person continues in his false beliefs and through generations the numbers of false disciples grows exponentially from that point on. On the Contrary, you take the opportunity to proclaim the teachings of the Gospel as preserved and continued from the time to Christ and that person remembers at some time in the future the witness you made and in part that witness leads him and generations of others to the Truth that is Christ Jesus through his Church.

This Cascade effect will be made known on the last day. Not just where evangelization and apologetics are concerned, but how every word, act of kindness, every direct sin forgiven or not, every thing that you have done which brings people to God or away from him shall be revealed.

As we continue our Christian walk as Catholics, we, like Peter who made a mistake, must grown in sanctification as we get closer to our Particular and General Judgments, we must become better Catholics, more humble more repentant sharing our faith and by so doing grow the body of Christ which is his Church.

Catechism of the Council of Trent (The Roman Catechism) "Article 7: From thence he shall come to judge the living and the dead.:"

Reasons For General Judgment

It is necessary to show why, besides the particular judgment of each individual, a general one should also be passed upon all men.

Those who depart this life sometimes leave behind them children who imitate their conduct, dependents, followers and others who admire and advocate their example, language and actions. Now by all these circumstances the rewards or punishments of the dead must needs be increased, since the good or bad influence of example, affecting as it does the conduct of many, is to terminate only with the end of the world. Justice demands that in order to form a proper estimate of all these good or bad actions and words a thorough investigation should be made. This, however, could not be without a general judgment of all men.

Moreover, as the character of the virtuous frequently suffers from misrepresentation, while that of the wicked obtains the commendation of virtue, the justice of God demands that the former recover, in the public assembly and judgment of all men, the good name of which they had been unjustly deprived before men.

Again, as the just and the wicked performed their good and evil actions in this life not without the cooperation of the body, it necessarily follows that these actions belong also to the body as to their instrument. It was, therefore, altogether suitable that the body should share with the soul the due rewards of eternal glory or punishment. But this can only be accomplished by means of a general resurrection and of a general judgment.

Next, it is important to prove that in prosperity and adversity, which are sometimes the promiscuous lot of the good and of the bad, everything is done and ordered by an all-­wise and all­-just Providence. It was, therefore, necessary not only that rewards should await the just and punishments the wicked, in the life to come, but that they should be awarded by a public and general judgment. Thus they will become better known and will be rendered more conspicuous to all; and in atonement for the unwarranted murmurings, to which on seeing the wicked abound in wealth and flourish in honours even the Saints themselves, as men, have sometimes given expression, a tribute of praise will be offered by all to the justice and Providence of God. My feet, says the Prophet, were almost moved, my steps had well nigh slipped, because I had a zeal on occasion of the wicked, seeing the prosperity of sinners; and a little after: Behold! these are sinners and yet abounding in the world, they have obtained riches; and I said, Then have I in vain justified my heart, and washed my hands among the innocent; and I have been scourged all the day, and my chastis*m*nt hath been in the morning. This has been the frequent complaint of many, and a general judgment is therefore necessary, lest perhaps men may be tempted to say that God walketh about the poles of heaven, and regards not the earth.