It's for the same reason people buy Ultimate clears; Wealth is a flex. It lets you pose as a benefactor to people and do things to feed your ego. A lot of people that buy gil constantly give it out, or buy things for people to force them into weird transactional relationships. I've known people like this over the years.

As for what they spend it on, look no further than the new Criterion mount. Hundreds of players are irrationally shelling out anywhere from 50-150mil for an ugly mount, and I guarantee a lot of the money moving around is tainted.