What would be the underlying reason to hold onto it?
The reason is that fiat money is being manipulated and its value going south and basically it's a tool of the enemies of humand kind: the banks (central and commercial ones).
Having one's money in the bank gives power to the criminals.
This has nothing to do with operating a business.
Almost all businesses have to pay their suppliers and employees with fiat. They simply can't hold the BTC they receive as payments, except perhaps for the small part that constitutes their profit. So for now, the use of payment processors that convert BTC to fiat is perfectly logical for companies wanting to accept Bitcoin-payments.
I agree that in an ideal case, a company should be able to do all of its business in BTC and not have to convert to fiat, but the ideal case and reality are miles apart for now.