What does reporting for leaving/afk do?

The only thing it really does is add to the very small possibility that the player will be manually reviewed. The Leaverbuster has nothing to do with the report system and will still punish players whether they are reported or not.

What does reporting for leaving/afk do?

Nothing.

not much. since i had a j4 that died in my promos 2 times and then rage quit in less than 10 mins.. and he's still playing the game like nothing happened. sooo it does nothing . inb4 fan boyz come in ignore them they're robots [zombie-brand-clap]

No I think it just gives em leaverbuster still. Reporting for afk, I've heard, is for when they dont literally disconnect from the game, but are just going afk by standing there doing nothing. Leaverbuster cant detect that unless it hits a certain amount of time, so you need to report it.

Afaik people who leave games on regular basis get punished. In reality though only players with bad internet connection receive punishment. Those who just ragequit... well... you probably guessed it already

I use it when someone disconnects after saying something of their intentions beforehand (I'm done, I quit, etc) or if they disconnect right after a failed ff.

The leaverbuster might catch most of it, but the report might help identify patterns for bigger punishments.