Explain the prosecutor's fallacy like I am five.
Imagine you have a bag of 100 marbles. You know that 90 of the marbles are blue and 10 are red.

You pick a marble out of the bag without looking, and it's blue.

The prosecutor's fallacy is when someone argues that the probability you picked a blue marble is 90% or very high. But that's wrong.

Once you've picked a marble, either it was blue (which it was) or red. The probabilities reset to either 100% or 0%.

The correct way to think about it is that before picking, there was a 90% chance of picking blue. But after picking, there is a 100% chance that you picked the marble you picked.

The prosecutor's fallacy gets its name because prosecutors sometimes commit this fallacy, arguing that the probability of someone's guilt is very high based on evidence, without taking into account that either they are guilty or not guilty, 100% or 0%, once the crime has occurred.
Why would a prosecutor want to perform a fallacy if they are interested in justice?
Fallacies aren't necessarily malicious. They are usually mental shortcuts that can lead to a wrong conclusion. The reason they are so common and dangerous is that they are often, but not always right and certainly not inherently right.
A prosecutor might unintentionally use it as a mental expedient to avoid using mental power on the problem. They are often overworked. If it's being done maliciously the least bad reason would be that in the US at least almost all court cases lead to a conviction of some sort. Sometimes this involves gamesmanship on the part of the prosecutor and plea deals, but often the court system mainly takes up cases with overwhelming evidence (such as caught-in-the-act drug possession cases) and the prosecutor gets into the mindset that the accused is always guilty. Finally, the possibility exists that the prosecutor simply isn't interested in justice.