Casual fans fall in to two types.
First are those people that will watch anything sports-related, regardless of what it is. These guys were kings back when there were three broadcast networks and Wide World was showing cliff divers, log rolling, and Topo Gigio. The rise of cable sports networks and regional coverage pretty much killed this type of casual fan off. Instead of being stuck with the Lithuanian Bench Press quarterfinals, the fan now had options to watch the sports that interested him or her.
Second are those people who will watch 'major events' just because everyone else is. These are the people who don't know squat about football but host Super Bowl parties, go to the Kentucky Derby but never see a horse, or watch the Daytona 500 because a woman is on the pole. These are the casual fans that 'storylines' can attract. The problem is these folks are usually interested in the gimmick du jour and rarely become week-to-week fans of a sport.
EDIT: astonishingly, I spelled 'Topo Gigio' correctly on the first try.