With just a single file you can trivially use it such that everything is inlined (if it's of the sort that static-s all functions, at least), even across multiple files using it, without needing the full compile-time-destruction of LTO.

And generally it's one less file to look at, more easy to copy-paste into your project (and as a very minor security benefit you'll potentially look at arbitrary subsets of the contents every time you do a go-to-definition or use the header as docs (thus having chances to notice oddities) instead of just looking at a header).