As Poppy says, people turn up and don't book, which can mean long waits if you turn up at the wrong time. I would book if I was you. If you then wanted to change the booking you can call 33 1 60 30 40 50 to check to see if there is another time.
I would share an experience I had at one restaurant and this is a useful tip. We went to the Restaurant des Stars just before 1pm, which we had not booked - the restaurant was about 1/4 full only, if that. The waitress asked us if we had a booking and as we did not, she pointed us to a long queue to the side of the restaurant, just inside, and curtly said that if we had no booking we would need to join this queue and it would be a 45 minute wait. It seems they drip feed in people that have not booked.
So I called 33 1 60 30 40 50 and asked if I could book a table for 1pm - no problem, plenty of tables, Sir. So back to the waitress again and I smugly said, oh I have a booking for 1pm, despite not having one 5 minutes earlier! She checked her screen, and there it was. The look on her face was priceless!