In the USA, there are something like 20 states that have NO major league teams in any sport (and that's states, not counting where there are multiple major cities in the same state.)
Then off course there are hundreds of decent sized secondary cities.
So you can in part understand why collegiate sport is big. But its also means that 2nd or 3rd tier teams can actually generate their own support in the right city.
I don't know about NFL, but MLB has used a feeder system of minor-league teams for a LONG time, and it works well for them.
I think for development of refs, its probably a good thing.
I don't know about coaches? I imagine most come from the blue-chip collegiate programs?
Obviously also, to some extent, its a good intermediary step between College and the NBA. Not every draftee is going to be good enough to debut immediately. Then you have guys like Ennis and McRae, who obviously benefited from extra development, maturity, and experience.