I had a friend in town from Denver Colorado next weekend and I wanted to plan one dinner out that was really nice.  I was hoping to find a high end seafood restaurant that had availability for us at short notice.  I searched yelp until I found a place that I thought would be absolutely perfect.  The restaurant is called Grill 23.  I thought for sure that they would be booked but I decided to try and book it anyway.  I accessed Open Table on line and submitted my request for Grill 23 at 7:00 on Saturday night.  Unfortunately they were booked so I had to make our reservation for 9:00 instead.  On Saturday my friend and I spent the day touring and shopping around the city.  By the time it was time for our reservation we were very hungry.  We went to Grill 23 and ended up having an amazing dinner.  It was definitely worth the wait. 
Question: Why did they ask for the reservation?
So they could guarantee they'd have a table.