Rust compiles to LLVM IR. I'm pretty surprised that building this transpiler was considered a better use of time than writing an LLVM backend for whatever "weird embedded target" might need this.
If you write a library in Rust and want to make that library available to other language ecosystems, not requiring a Rust compiler toolchain for using the library is a pretty big plus - instead create a C source distribution of the library, basically using C as the cross-platform intermediate format.