C or Python, if, when and why?

C for a single platform... Python for when you want to multiplatform...

If you need high performance, C (or the fast bits in c). If you need easier Dev, python

Cython!

Scientific stuff: combination of c++ and Python (think cython or 3rd party packages like Boost)
Visualization, analysis, parsing, data cleansing etc: Python
Real-time tasks: c/c++

To answer your why.. Python is syntactically beautiful! Easy! It does all the hard work for you but that comes with some overhead which is acceptable in most of the cases unless you're doing something that requires you to manually take care of memory or something that is real-time like an app for stock market which updates data at milli to micro seconds scale.

To me C is more neat and tidy thank to the curly braces(dunno what they're called). Python a bit messy and inconvenient due to the strict indentation.

In 2016 using c as yout default language is a bit weird.

Sometimes you specifically need it.
But if you don't there are better options