Good question, it's true that at death judgement begins. One reason I can think of why prayer is necessary is that the few souls that do go to heaven, don't go straight to heaven. You would have to be Saintly to go straight to heaven. Instead, they go to purgatory first. Purgatory is very painful and the amount of time spent there depends on the amount of purging a soul needs before they can enter heaven. Souls that committed much evil in their life, but did very little penance spend a longer time in purgatory. There are souls that will be in purgatory until the general judgement. Some souls may only spend 30 years there. So offering prayer for those that died helps reduce their time in purgatory. "And if any man build upon this foundation, gold, silver, precious stones, wood, hay, stubble, the work of every one shall be made manifest: for the day of our Lord will declare, because it shall be revealed in fire : and the work of every one of what kind it is, the fire shall try. If any man's work abide, which he built thereupon : he shall receive reward. If any man's work burn, he shall suffer detriment : but him self shall be saved : yet so as by fire." 1st Epistle of St. Paul to the Corinthians Chapter 3, verses 12-15 Here's a interesting read on what happens to the soul at death: http://www.catholictradition.org/Classics/4last-things.htm