• Back in the day, it used to be cheaper to buy season tickets than to buy tickets for every individual game. So a whole lot of if it inertia from being the standard way to go to all/most home games.
• Knowing you have the same seats in the same location near (mostly) the same people for every game it pretty nice.
• You can just relax with the comfort of having tickets in hand before a game instead of regularly having to go through the hassle of finding decently priced tickets before any game you want to attend.

But yeah, the administrative need to maximize revenue in the next quarter and the enshittification of everything means these benefits are hardly worth the cost any more. Which is why we've seen lots of articles the past few years about decreasing attendance in stadiums nationwide. Surely it must be because the fans aren't supporting the teams hard enough any more!