It's more important to look at why the rich might be a problem than why their money might be useful to us. I know a lot of non-billionaires spend their lives defending them, but there is a case to be made for why their industries and disproportionate social power have been hurting society and a wealth tax is a small way of balancing out wealth inequality which, yes, is a bad thing, and you can do a thorough read of wikipedia if you don't think it is a bad thing.