It takes C or C++ code...and compiles it.
You see, if you actually knew what you were talking about, you would know that devkitpro/advance generates HUGE and very terrible code ( as far as assembly and instructions go atleast ). Even optimizations don't do that much for it.
What this thing does is makes actual optimized code. IDK if it does anything about file-size compared to devkitpro ( I haven't used this thing just yet -- still -- ), but it definitely increases over-all performance of the .elf files generated. That's my comprehension anyways.