Maintainability. Same reason that, when you code, you want to include comments, have good variable names, use reasonably-sized functions, etc. It makes mainta

Maintainability. Same reason that, when you code, you want to include comments, have good variable names, use reasonably-sized functions, etc. It makes maintainability and future extension much easier. Forget about semantic purity. CSS has survived all these years because it's been very useful in a very practical sense.