Hoping others with intimate knowledge of investment banking and the market can also chime in, but here are my thoughts.
1.) Raise substantial additional capital for R&D, growth, new products, pay down debt, etc.
2.) Exit event. Allows founders, employees and pre-IPO investors to "cash out". Typically at nice fat returns since they get heavily discounted shares.
3.) Increased public awareness since IPO's usually have media coverage and a general frenzy.