I know people who do it for a living. I'm not being a dick. If the finDom made them spend the money on something other than themselves, then I could see it being a legitimate kink (I'm sure some exist) but mostly what I see is an extortion racket or gold digging. Which, hey if that's your thing, more power to you, just call it what it is.

Now, as for why the finsub does it, the reasons are varied. Some have massive amounts of disposable income and the love playing sugar daddy, some have had their lives so controlled they need it to feel "normal", others unfortunately are just suckers.

I'm sure there are people who find this type of thing fun, and just like anything else, literally anything can be a kink.