Why do firms go public?

In a piece of fortuitous timing given that last night in class we began a section on raising capital and IPOs, today's NY Times DealBook mentions two firms that are selling shares so that the firms' current owners can cash out a portion of their shares.

1. From the NY Post: "National Lampoon Inc., the media company that cast comedian John Belushi as an irreverent fraternity boy in "Animal House" in 1978, is planning an $8 million stock sale designed to raise its profile on Wall Street and finance the buyout of a former owner." As an aside, the hope to list shares on the American and Pacific exchanges.
2. And from the Guardian: "Stead & Simpson, the British shoe seller that can trace its lineage back 171 years, is considering a return to the London stock market as its main shareholder looks for an exit."

This is not to say that a desire to cash out is the only reason firms go public, but it is one other reason.