There is no guarantee that the stock price will immediately go up, but when it does the underwriter stands to make a nice profit. With only a few exceptions the IPOs of high-tech firms in the last year have gone up substantially. IPOs of high tech firms are the most lucrative kinds of IPOs on Wall Street. In most IPOs the underwriters, which are usually large brokerage firms, may sell some or most of their shares their preferred clients or customers at the initial offering price. This is considered a perk.
Brokerages use it as a perk to retain their customers.