This call transpiles the Stan model to C++. If you ignore the generated C++ its basically a syntax check.
This call is also the first step when you run
make model then uses the generated C++ together with the rest of the Stan core to make an executable. A call to bin/stanc ends with a generated c++. This part is really fast and can be used in this context.
bin/stanc model.stan --auto-format formats your Stan model (the output is a formatted stan model) but it currently does not preserve comments unfortunately.