It says it's a C frontend for `zig translate-c` which translates C code to Zig code. `zig translate-c` is not required for most Zig projects or even for most Zig projects that interop with C.
I also started reading through the Zig overview [1] and near the end of that document it says this: "Not only can Zig compile C code, but there is a very good reason to use Zig as a C compiler: Zig ships with libc."
From the doc, IIUC, compilation of header files and other C code allows Zig to do direct interop with no intermediaries. Seems useful.
I guess since you said header, you keep everything opaque and create a header for that which gets translated to Zig.