A-Attack in lane

Been watching some people play league on youtube. I notice a lot of people A-click every minion instead of just right clicking. Can someone tell me why they do that? I could understand the whole "miss minions and walk into enemy" thing with right click, but I cant remember the last time i actually missed clicking a minion (Like really, if you miss stationary minions, how do you land skillshots). And even if you DO miss the minion, not like you cant just move yourself back withing 0.1 second right?

So why the whole A-click thing? Some secret thing i know nothing about?