Yes, it may appear a bit esoteric. There are a number of reasons why not all the seats are sold, some of which have been listed above. There comes a point near the date of the match when there are a lot of single seats left. These are difficult to sell. Suppose there are a couple of thousand unsold tickets on match day morning, who is going to travel to the ground in the hope of getting one? This is a problem with all-seater ticket-only games, you can't just cram the last few thousand in at the back.