Modularity also allows for code reuse. It increases maintainability.
another thing to think about is how this was not invented by frontend frameworks. We did it fine pre-SPAs and pre-preprocessors. It was part of the architecture and strategy. The hard work that allowed us to essentially reskin entire, very complex, projects every couple of years
i’ll put myself out there - here’s a receipt from 06~07 https://web.archive.org/web/20070512035940cs_/http://www.toyota.com.au/toyota/main/css/elements.css
we were a team of 5 devs including me. We weren’t tribed off into separate areas of concern, we all knew the whole project back to front, and (maybe not the most clever move) managed without version control by always being aware which part we were working on. Cos, ya know, communication is easy when you are 5 people sitting in a group.
Don’t give me shit about the complexity of the UI in modern apps either. We were dealing with a huge collection of brochure style pages that had plenty of variations. We kept all that css under 500kb. We could achieve the bland flatness of modern uis under 100kb easily. No fucking doubt.