Given a legacy C program it may be desirable to maintain the main() function in the legacy code, and let it call out to a Python interpreter. The C code can define Python functions and variables to allow Python to manipulate things in the C world. In this manner one adds scripting to a legacy C program.

This is almost certainly not the approach you'd take with a new program. The preferable approach would be to write everything in Python initially and translate pieces of it to C only where necessary for reasons of performance, or security, or access to resources not directly available in Python. So the decision about which pieces to translate might be made with the help of a profiler to identify the slow pieces, and a reference implementation might be maintained in Python for purposes of regression testing.

Similar considerations would apply to other higher-level-than-C languages such as Lisp, Scheme, or Java.