how do i know when to use it and when to not?

If your audio is mixed correctly and sounds the way you want it in the render, don't use AGC.
If you've got raw camera footage with loud louds and soft softs, it may give some basic improvement if you're unwilling to compensate at the track or event level with normalization, compression, or limiting.
In other words, don't use it.

If you choose not to use it, you'll be right somewhere between 99.99% and 99.999999% of the time.

Since you're merely re-encoding a previously finished work then not using it is right at least 100% of the time. You don't want to mess up what someone else with probably much more experience spent a lot of time on getting right to begin with.

When would you use it? Maybe if you have very amateur material with very bad mic placement so that some voices can barely be heard while others are booming, it can help even out the difference. It will also make the audio very flat and lifeless. This task is much better handled with the more powerful and flexible tools in Vegas than with a simple AGC.

Hmmmm. If great minds think alike, what's our excuse?

Guess our radar is working tonight, eh Kelly?
Essentially the same response, five seconds apart.