This is C interop.
I work with C quite a bit and I enjoy it, however writing a large project in C can be tiresome.
Having an option like Zig which can import C headers and call C functions without bindings is pretty attractive, especially when you want to write something a big larger but still stay in C world.
It's also the source of my major problem with zig. It doesn't have its own ABI [1].
So, if for example, you want to write a library in zig, to be used by others from zig, they must build your library with their project. That may not be an issue for smaller things; but for a large library I'd really like consumers to be able to pull in a binary with just a definition (header) file. Since zig uses the C ABI, that would currently mean translating everything to and from C at the binary interface, and losing all ziggyness in the process.