Why not? You keep up to date, experiencing world-class hiring process and its requirements (IMO FB's hiring process is the best I've ever experienced, Google's feels one level down), you meet interesting people you might later work together on something cool either there or at another company, you visit interesting places, get some inspirations for your own businesses, interviews might be also a lot of fun (one interview I had at FB was an hour of one-joke-after-another while discussing/inventing architecture of some scalable system; imagine Cryptotinder which is a joke on its own) etc.