It makes it easier to relink against a different standard library like musl. Or for cross compilation. The fewer files to deal with the fewer places I have to look for issues in porting.
I mean, you're already going to have to recompile stuff for either of those cases. Once you're compiling things, compiling several files isn't much harder.
Right, it is almost always possible to do anything. In practice I would rather have to change the includes on one file to get it to compile with musl rather than change the includes on 10+ files each with a different change. It is a matter of degree not kind of difficulty. But in general I always will prefer the issues that arise from a single file implementation rather than the issues from many files.[Sorry if this is phrased poorly, I am not a native speaker.]