By piggybacking off GCC et al you gain very easy portability/access to a bunch of platforms that most languages would never attempt to support.

We used this in production with Nim for embedded firmware engineering at my previous job doing industrial IoT systems, which let us write a much nicer language than C (and much faster, much safer), with code-sharing between the network server (and its comms protocol) and the firmware code itself.

All can be done with C itself of course, but this let us achieve it faster and in a much nicer fashion

Not strictly for compilers (which probably don't need to use macros much), but for normal macro-codegen it would be very useful to have some some way to add line returns in macro-generated code so that it's easier to inspect with gcc -E