You still have to compile the library each time you recompile whichever source file includes it. If the library had its own source file, you would only have to

You still have to compile the library each time you recompile whichever source file includes it. If the library had its own source file, you would only have to compile once and link it thereafter. This adds up if you have a lot of header-only libraries in your code base, and the total LoC may very well dwarf your own codebase. This is where precompiled headers come in, but introduce their own downsides.