People buy tickets on the flights they want to be on. And unless it's on a route with very frequent service, it makes no sense for someone to go to an airport several hours early to standby on a flight that they may or may not get on. People standby because there's some change in their travel plan, and/or they get to a connecting hub earlier than they anticipate.

Airlines let people standby because there's really no downside to them. They can still sell that seat all the way until just before door closes, and then clear the standby list. At hub airports, they need to get people on earlier flights so that there will be room in the later flights for those who had missed connection to the earlier one.