College athletics is a pretty good system for promoting lesser-followed sports that would have difficulty maintaining an organized league -- team volleyball, swimming, field hockey, rowing, etc -- because it provides infrastructure and funding that would unlikely be available if reliant on revenue.
On the other hand, "amateur" leagues for popular sports (football, basketball) being associated with colleges is extremely effective at amplifying the interest of these leagues. Consider how little interest there would be in the "Ann Arbor Wolverines" vs "Columbus Buckeyes" in the USFL without any association with schools and their alumni, or whether anyone outside a 20-mile radius would watch Durham vs Chapel Hill if they played in the NBA's G-league.
On the other hand, "amateur" leagues for popular sports (football, basketball) being associated with colleges is extremely effective at amplifying the interest of these leagues. Consider how little interest there would be in the "Ann Arbor Wolverines" vs "Columbus Buckeyes" in the USFL without any association with schools and their alumni, or whether anyone outside a 20-mile radius would watch Durham vs Chapel Hill if they played in the NBA's G-league.