Seriously, does that really do something? Is there any official note about what does this feature do?

Riot hasn't commented about it that I've seen, except to clarify that it was NOT included in the ban wave for report abuse last month.

Personally I think it's a placebo button. Does the same thing as the crosswalk button at a big city intersection or the door close button on an elevator. At most, Riot collects data from it to gauge how well the matchmaking is working.

It's an obvious placebo button to cut down on rage reporting. If you want to push someone into the Tribunal faster where they might be punished for calling someone a noob in some other game then choose "Intentionally Feeding."

definately placebo but i use it every other game cause the player base is so bad.

There was an official statement regarding this several months ago. Can't find it and don't remember the exact wording, but it functions as a way to never be queued with that player again.

That is its only function and the reported player won't ever turn up in the Tribunal for this selection either.

Doesn't it lower the player's ELO?

I actually heard that was the way it worked. If a player accumulates enough of them, it will lower their normal (not-ranked) ELO slightly. Not sure if that's accurate or not though.