With any programming language, when you do some client or day job work, you need to deliver quality software. When you contribute to FLOSS projects, you have to follow some style guides specified for that project. Great help with all of that are linters and style checkers.
Every time you call `load-theme` in GNU Emacs, there is a chance that previous theme changed something that new theme does not. Also, sometimes themes change `linum` (line numbers) settings and if you use `linum` you don't want that.