remember that there are different reasons to make reservations.

Sometimes if the place is busy OR darn near empty...they can plan staff

Some places, cook to order (you order at reservation)

Rarely is the case off season where there are no seats available. However, in high season 1,2 or 3 of the above may be the case.

Either way, if you don't make reservations, you risk getting a table, and ZERO service.