What is the point of the PBE?

So, with all the bugs going around, like incredibly bag lag or this lobby bug to where you can't pick a champ or change a summoner spell, making it to where you literally have to leave the game and get punished for it, I have one question. What is the PBE for again? Isn't it made to find and execute solutions for bugs for the live servers? I can understand there being issues right when it's released live, but this shit is literally game breaking and I literally have to wait for FIVE 20 minute timers because of this. Not only that, but you guys have polished it like 3 times already? I just don't see the point, once you KNOW it's polished correctly, then post it, instead of punishing players who have done no wrong.