My suspicion is that this is currently more art than science; the banks/IPO underwriters want the stock to pop, aside from even the guaranteed floor you mentioned. They want the psychological effect that comes with that "pop", that shows up on potential investors' radar.
On top of that, I imagine it's a little tricky because an IPO is when the early investors cash out, so they're the largest stake-holders in the company, and creating that "pop" in the hope that people buy-buy-buy to drive up the price creates a good return for those investors/stakeholders. I don't like it, since it can essentially screw the company out of cash, but that's my guess as to why such a thing exists.
On the other hand, a decent number of companies going public these days are going to screw over the investors anyhow (cough Zynga, GoDaddy, King), so if that means they get slightly less cash in the bank, I guess I don't really care.
On top of that, I imagine it's a little tricky because an IPO is when the early investors cash out, so they're the largest stake-holders in the company, and creating that "pop" in the hope that people buy-buy-buy to drive up the price creates a good return for those investors/stakeholders. I don't like it, since it can essentially screw the company out of cash, but that's my guess as to why such a thing exists.
On the other hand, a decent number of companies going public these days are going to screw over the investors anyhow (cough Zynga, GoDaddy, King), so if that means they get slightly less cash in the bank, I guess I don't really care.