What's the benefit of this?

Say I am writing a transpiler to C, and I have to choose whether I will target C89, or some arbitrary subset of C23. When would I ever choose the latter?

The only benefit I could think of is where you're also planning to write a new C compiler, and this is simplified by the C being restricted in some way. But if you're doing this, you're just writing a frontend and backend, with an awkward and unnecessary middle-end coupling to some arbitrary subset of C. What's the benefit of C being involved at all in this scenario?

And say you realise this, and opt to replace C with some kind of more abstract, C-like IR. Aren't you now just writing an LLVM clone, with all the work that entails? When the original point of targetting C was to get its portability and backends for free?