I don't get people watching single player games but I watch people play competitively to keep an eye out for new strategies and I also have some teams I'm a fan of.
It's not very fun watching someone play Fallout, but it's entertaining watching someone play Fallout role-playing as a homicidal maniac that uses nothing but his bare fists. The sheer difficulty is impressive, and if coupled with a talent for presentation/humor it can be a pretty fun time.