Why would a business with most of its income in the form of BTC rather than $, elect to exchange that BTC to $ to pay rather than just paying with BTC directly?
The dollar price is fixed when services are rendered, and due 30 days later.
The vendor charges more in bitcoins to cover their costs in converting them to fiat
Taxes
The dollar cost is the same month to month, while the BTC cost fluctuates with the exchange rate