A large part of this is allowing their employees to cash out. When a company goes public, there is often a clause that prevents sales for a certain number of months, and who knows what the share price will do by then. This is actually a really good thing for Unity to do, trying to make the "stock compensation" perk into a meaningful cash-out. It also means employees don't have to deal with holding shares in a public company if they don't want to, as that's hassle.
