Airlines would love it if every passenger would reserve their seat as far in advance as possible. That way, they'd always know how many flights they actually need for each route. So they make it as attractive as possible for people to book early. To punish procrastinators, ticket prices get higher as take-off gets closer.
Up to a point, at least. In the end, the airline just wants to fill every seat. So, if there are a few seats left open at the last minute, you can sometimes find a bargain deal. If you really have to fly, though, don't count on that. Airline bean counters have gotten pretty good at knowing just how many seats they need.