Same way politician candidates often acquire some military background. It builds their profile and they have something to tell stories about. It doesn't really matter what they actually did there.
In modern times, most people at least in the US doesn't have military experience and they tend to give it a somewhat mythical status. Those who do, can feel that the candidate relates to them.
So it can be an rhetoric for anything "you know, the time was I went around moon, this happened, which made me realize that x is more important than y". No-one can really discount that experience easily or politely since they haven't done that, and somewhat more admirable and mythical than creating a social networking website.
He clearly has political ambitions, but he must acquire personality. Being an space-tourist would help.