A good example of where Python’s useful is all those “import/export” scripts. You don’t need C/C++ code to do that. It’s something that many users would like to (and do…) “tweak just a little bit.” By having a Python-enabled interface Blender is able to let you do that very easily.
The C/C++ core-system provides the implementation of the Python objects and the system’s reaction to them. But it doesn’t have to be burdened down, within the core code, with fifteen or twenty different niggling variations on “import and export.” Furthermore, once the core interface has been implemented and done very well, once, as it has been, then you can apply it to dozens of different uses … games, tools, you name it. It’s certainly one of Blender’s most powerful features.