It makes sense as a porting tool, if you need to port the Rust compiler to some target that has a C compiler.

But it doesn't mean rustc generates code for that target, only that you can run it there. You'd still have to teach LLVM about the target. Although that might already have been done.

It's not that useful for retro computers because the Rust compiler needs too much memory for most machines of the 32-bit era.