The advantage of general admission is that, instead of having the same fan sat in the same seat every game, it opens up that opportunity to multiple supporters. A season ticket holder might buy a shirt, or other merchandise, at the first home game of the season and never go back the club shop again until the following year. However if over the course of 20+ games that one seat ends up being shared by several different people then that in turn increases the likelihood of more 'once a season' club shop visits.
Furthermore whilst season tickets provide a steady income stream they again restrict the overall turnover of fans. A greater turnover of fans helps create a wider fan base and gets more people, with money to spend, involved with the club. Plus general admission seats usually cost a couple more quid than equivalent price of what a STH pays per game.