If a guest takes the time to come to your website to book a table, only to find that there is no availability for their chosen date / time, you may be losing some valuable revenue.
As you already have a guest willing to make a reservation at your restaurant, it only makes sense to offer the ability for that guest to add themselves onto your waiting list, should a table become available, if any of your existing booking makes a cancellation.
A quick call to the guest and push of a button turns the waiting list enquiry into a booking.