Because reservations affect popularity, both in general and with specific, potentially desirable customers. All else being equal, I am more likely to eat at a

Because reservations affect popularity, both in general and with specific, potentially desirable customers. All else being equal, I am more likely to eat at a restaurant that takes reservations than one that does not because I know I'll have a table when I get there, and I doubt I'm the only person who feels that way. This is particularly true for higher end restaurants where the "experience" is a bigger part of meal. And waiting around to sit down is probably not the experience you had in mind. And it's worth noting that a reservation is valuable even if there isn't always a wait. Unless the customer is a regular, he's unlikely to know what the wait would be like in the first place, and things vary depending on the day and time. A reservation removes that uncertainty for the customer, which is valuable even if a reservation wouldn't have been required.