what's the point of "leaving the game afk" report option, if there's leavebuster?

Leaverbuster handles DCs and ragequits with LPQ. It is entirely automatic and relies only on timers.

Other types of going AFK, like dancing/typing in base, or farming the jungle for the purpose of not contributing to teamfights need to be reported for the IFS to investigate and punish (with a 14 day for the first offense, as it is a gameplay offense).

Leaverbuster is a bot Therefor it can be cheated/miss some

If someone were to get a large amount of those it could potentially lead to a manual review.

Good question. I think its mainly for those who arent technically afk (as in, away from keyboard) but arent participating in the game. Like walking around base/fountain and stuff. Though, that is essentially griefing.