The vented bbl is a takeoff on the King aftermarket vented rib for target shooting. So the original idea with vented bbls as add ons was to add weight to the bbl for target shooting. You could do the same thing by just making a bull or heavy bbl, and mostly that was what was done for production guns, but it looks nowhere near as cool.

See page 402 of Wilson's The Book of Colt Firearms for a prototype.

The most entertaining explanation I have read of the Python vented bbl is that it was for heat dissipation.