Hmmm maybe to gather large amounts of data and to make sure that a lot of the bugs are worked out. As far as the balancing thye can get more data in a week of pbe then they can in several months of internal testing. Once they have the data it has to be collated and studied. once that is done then the balancing can begin. However how to balance a champion is a delicate procedure as you wan to keep their identity. this has made several champions hard to balance......
The balancing can begin. However once they determine a way to balance a champion they then have to test their changes. And thus starts the whole mess all over again. However expecting them to scrap a entire rework is beyond idiocy in most cases. Pbe is not about the community shaping a champion it is about working out bugs and gathering data for balancing.
Now keep in mind a week on live is like a month on pbe so balancing take a while.