Separation of concerns is a really important concept, but I also think it's very often misinterpreted. The real question is where are the inherent couplings wi

Separation of concerns is a really important concept, but I also think it's very often misinterpreted. The real question is where are the inherent couplings within a system? In places where they exist, collocation is a very good thing. Separation of concerns is best applied parts of the system with low inherent coupling. In the case of web UI, the controller logic, document structure (HTML), presentation (CSS), and client-side interaction logic (JS) often have very tight inherent coupling, to the point of needing to relate to each other through shared identifiers (e.g. DOM classes, variables). Collocation generally allows you to scrap boilerplate declarations and work on an entire UI component in one screen of source code, rather than bouncing between tightly coupled files. In my experience, this is a huge productivity boost and aid in understanding code other people wrote.