The main reason, for me, was that i got the actual price at the time of ordering, regardless of the price when paying (usually later, depending on the payment method).
Second reason was that the price at the broker got averaged and updated every 90 seconds, which allows to catch dips while the prices already goes up on exchanges already.
Something like an arbitrage-proximate time machine  

Another bonus, whichever address the customer sends the Bitcoin to, if it's his exchange wallet, the original fiat buy is tainted.
And of course, you don't need to have money lingering in your exchange's fiat wallet to make a quick buy (preferably if bitcoin price goes south).