The idea of mounting a Python barrel on a S&W started with the the Practical Police Course shooters.
Many of them preferred the S&W trigger, but liked the balance and accuracy of the Python.
Fitting a Python barrel gave them the best of both.
The Python barrel was more accurate both due to it's faster rifling, but also due to Colt's superior quality barrels, and due to the fact that the Python had a tapered bore.
The bore actually tapers toward the muzzle, getting progressively tighter toward the muzzle.
This grips the bullet tighter as it moves down the bore, and provides better accuracy.
After people saw the "Smolt's" or "Smython's" being used by PPC shooters, the idea of course started being used on non-PPC custom guns, just to be different.
In addition to the Python-S&W conversion, there was also a Python-Ruger version know as a "Cougar".