[racket] Compiling C to bytecode

I'm looking over compiler collection that implements a Racket->C compiler.
 It looks like it uses a set of racket structs labeled vm:* as the virtual
machine representation, and then compiles these to C.  However, the main
racket compiler uses a different bytecode representation, implemented in C,
that runs on the racket VM.  Why are there two different bytecode
representations?  Isn't it a duplicated effort to have a different
intermediate representation when compiling to C?  Why not first compile to
the main racket bytecode, and then compile that to C?
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