Private leagues, bookmarks, the Scoreboard, and player numbers alone make supporter more than worthwhile for me.
Sure, you can schedule competitive scrimmages against a "league" of others manually. But private leagues give you an extra game a week that you can play around with tactics in or play to win and see how you stack up against others.
Bookmarks can be done simply by bookmarking each page. The built in bookmarks track everything by ending time and break up the bookmarks into Teams, Players, Staff, and Games. Much easier for sorting players and TL prospects.
The Scoreboard is great for tracking multiple games at once. Gameday, I've got 9 games up top of the page that I keep track of. Sure, you can manually open up each window but this way it's all right there.
Player numbers are a great way to sort your team and ease up your roster settings each game. You can always do without this, but it's a nice quality of life addition.
Overall, Supporter has some great features, none of which couldn't be done other ways. However, having all these quality of life updates in the game makes it much easier to use and thus more enjoyable overall.