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.
It only makes gtk theming slightly less of a pain in the ass, but you can use gtk’s inspector (enabled thru a gsettings terminal command and then ctrl+shift+d in the app or thru an environment variable) to find the classes, names, and object types for whatever you’re trying to theme.