1. It means you don't need C code & a C compiler in your project any more, which simplifies infrastructure. E.g. cross compiling is easier without any C.
2. You can do LTO between Rust and the C->Rust code so in theory you could get a smaller & faster executable.
3. In most cases it is the first step to a gradual rewrite in idiomatic Rust.
https://www.darpa.mil/news/2024/memory-safety-vulnerabilitie...