Show off to your other NPC friends you got money to spare, I think that makes you cooler

Why don't you purchase completely useless thing with your tax return money just to show others you are responsible with your money!

NPC of an RPG world is a profitable business. Do you get paid with actual gold? No...

You know you are a pro NPC when you can afford to buy such a useless achievement.

Its time to impress people don't like with money you don't have (maybe you do), but that doesn't matter.

I could save this money but instead you gotta spend it because that is what you do

You know how people buy useless art for millions? This is basically the same thing.