On Reddit, custom CSS makes a lot of things possible, allowing subreddit moderators to customize their subreddit, theme their subreddit, and introduce features Reddit may not have in general.

Without CSS, all subreddits would be the same as a default subreddit.

See https://www.reddit.com/r/ProCSS/comments/67j56f/css_isnt_about_themes/ about what else it does!