I cannot tell you how many times this question is asked in/on the blogosphere.
The best answer I’ve seen is that from a poster on CH, lemarais, who said “Why don't you want to make a reservation? Is there a downside to making one?” Perfect.
One makes reservations, even at a 100-cover joint turning tables (of which there are none here) because:
It is Courteous, Kind, Obedient, Cheerful, Thrifty, Brave, Clean, and Reverent,
It ensures you the best table, for me by the window not the bathroom or swinging door to the kitchen.
They know your name and despite the bad rep the French food folks have gotten, they will treat you as you treat them. Remember the Golden Rule folks.
It helps them judge the number of dishes they need to have on hand (not all places depend on massive Metro shipments or their "little guy" running next door for a steak or baguette.
As lemarais says "Why not?"