- you can build simple tools contained in a single .c file, and you dont't need a build system for this (e.g. just call "cc tool.c -o tool" instead of mucking around with Makefiles or cmake)
- the library can be configured with macros before including the implementation (e.g. provide your own malloc, assert, etc...), with the implementation in a .c file, these config macros must be provided via command line args, which implies a build system
- you can put the implementations for all header-only libs used in a project into a single .c file instead of compiling the multiple .c implementation files, this might speed up compilation
Single-header libs have some of the advantages that a proper module system would bring to C and C++ (e.g. extremely simple integration into own project and increased compile speed), but without all the complexity of implanting a module system into C or C++.