I spend a lot of time on the computer, using a lot of software. This makes me acutely aware when things don’t quite work right. A button that renders incorrectly. Inconsistent padding. Unthemed dialogs. Even the best themes out there had small bugs that annoyed me. I care deeply about a smooth, consistent desktop experience.
As far as i understand it (never made one, only modified), it’s like creating a global usercss for the web (that i did), except with imports and symlinks. You have some generally used class names that fit most of the apps, except that one tool and then some. Then there’s gtk 2.0, 3.0, 3.1, 3.1/4⁴, 4.0 and the window theme, which is (luckily still) separate.