Your C compiler is probably a very large C++ program written over many years and with millions of dollars worth of time poured into it. The C compiler I use takes up hundreds of megabytes of space in a minimal build.

So in my opinion it's interesting if someone can write something original with 80% of the useful functionality on their own, or achieve it in a different way. And their code might be more approachable/educational to people than the behemoth.

The benefit is mainly that it is interesting and easy to learn/experiment because it has a very small and tidy implementation..

Just an example, building cproc from source takes a few seconds - building gcc from source takes like 15 minutes at least.