Imagine that every Pi member is a bank that can lend Pi to other members without any intermediaries. The purchase of Pi by members equal to what they have mined has 2 benefits.
1. It gives flexibility on when to go public.
2. The Pi purchased by Pi members could be used as reserves similar to bank reserves. Individual members may be able to lend more to others based on their reserves. A Pi owned pay website to facilitate these high volume purchases could draw a lot of advertising and generate revenue.