Grigori Pereleman would be doing mathematics with or without the prize money.

I think sp4rki's point is that it's a waste if not taking/making the money means that you have less time to do whatever it is you're good at. E.g. the artist who could make money from it but chooses not to, and as a result has to work at the convenience store, and as a result produces less art. Though that last bit is a bit contentious - maybe working at the convenience store is a good source of inspiration for that artist.