The thing that's espcially nice about that interop is that Zig includes its own C compiler. That eliminates the pain of having a build script locate an installed C compiler and figure out what options should be passed to it.
Why would I use zig c compiler in place of gcc or clang? Mainly for zig interactivity or does it have some advantage other than that over the aforementioned compilers?
Zig's C/C++ compiler is just clang, but with header files for most major platforms included, and sane defaults, so there's no hassle getting it it to cross-compile. Some companies have been using Zig solely for an easier to use clang.