An IPO is an event, at a point in time. Think of it the same way you might think of a college graduation or a wedding. It's not the event that's important, it's the (relative) permanence of a degree or of a marriage.
If your question then is "What are the benefits of being a public company?" then liquidity, access to capital, and the ability to use stock as currency are three. The big negative is the cost of compliance.