Worth mentioning that some older editions of D&D had rules for acquiring and running domains in downtime, which was a big motivation and a way to spend your cash. So, your wizard could build a wizard tower, hire some guards, and spend their downtime cloistered with books inventing new spells and magic items. AD&D had rules for all of this (at least, 2e did).
Though how much this got used varied considerably, and often it was more a thing to do with all the gold you accumulated rather than something you accumulated gold for if you understand the cause-and-effect distinction I'm making (i.e. a number of characters would get into building a keep/running a business/etc just because they had all that money that wouldn't have sought out money just for that purpose).