There is a great advantage booking online, because if you cannot get the particular time you are looking for - it will automatically suggest alternate times or alternate restaurants. No offense, but it would take time for a human to do that.
If gives you ample time to make up your mind whether it is all about the time, the restaurant, or the experience. For me, I had one that I picked an earlier time, because it was important to go to; for another, it was all about the experience, and it picked an alternate location I would have never thought of.
The downside with the online system is that it allows you to make multiple reservations for the same time. We noticed when we went the week after Thanksgiving, some of the restaurants we had ressies in were only half full, but they were turning people away for walk-ups. The restaurants remained half empty the entire time we were there - but the reservations list was full. It points in the direction of people making multiple reservations for the same time and waiting till they get there to decide where to go.