They ask that you have a reservation, so that they can have the right number of buses.
If you're taking it to your hotel, you can pay ON the bus for a roundtrip, and then just hold on (for dear life) to the tiny little voucher they give you. That way you don't have to worry about it.
Though paying online gets you a small discount.
Anyway, you don't *have to* have the reservation, but they do like you to, so they know how many people will be on the bus.