I'd say necessity. I had been using a 10 speed that I've had for decades. It worked great for riding on a huge alluvial plain in the desert. Then I moved to the foothills of the Ozarks and needed a bike to climb hills. My ex had been given an old Specialized Hard Rock several years before and never rode it. It got pressed into service.
The bike got fenders, lights and a rack and wore it's old knobbies until they were worn out. 10 years on, it's been a dependable mount that gets me up and down the local hills.
