Why are relays so frequently driven by optocouplers?
First, a possibly more permanent link to this product is here. And the schematic is here.
(Edit 7/29/2015: Ironically my two links are now broken and OP's Amazon link is still useful)

Two reasons it makes sense to use optoisolators here:


The controlling device might be very far away so that it doesn't share a common ground reference with the relay board (except as connected through a long cable). Using the optoisolator means the control signal is used purely as a differential signal between Vcc and the control signal, both sourced from the controller circuit; ground potential differences won't affect the operation.
The relay coil voltage is not necessarily the same as the controller's Vcc. It could even be generated by an off-line (unisolated) supply. The optoisolator then provides isolation between the potentially unisolated JD-VCC supply and the controller circuits.