Changing the look of a website. Remove elements you don't like while formatting the rest in a more pleasant style.

In addition to what jayandp said, it can also be useful to correct some "bugs" if you know your way around CSS. For example something I really hate about the default Lemmy themes is that they stretch the videos to the full width of the page, so I made a CSS override to fix that:

video {
    width: 25vw !important;
}