[aetherson]

A couple of other commenters have basically mentioned risk hedging -- that is, the concept that you might think that your company is a good one that has better-than-usual chances to monetize, but recognize that even good companies fail. They're right, and that's one reason.

The other reason is liquidity. Sometimes you want money now, not in a few years. Maybe you need to buy a house, or you want to start your own business (and thus not have a salary income for the next two years or more), or you or your spouse are pregnant, or you just decided you have to have a new Model S.