There are probably almost as many reasons as there are fans.
The action, competition, danger, money, and bragging rights are just a few.
Some view their favorite players in a "Wish I were them" kinda way. They might fantasize about having the physical prowess, the mental toughness, the fame and/or money.
Some view sports as strictly an outlet for their work-a-day lives. On certain days, they can go and cheer on people/teams who they believe are representing their city against those who think they are better.
The danger of many sports (X-Games, Moto Cross, etc) will bring out many fans.
There are people who view sports as a job to pay their bills. They may enjoy their job (most do), but it is the source of income to support their families. There are 10's of thousands of sports jobs in the NFL alone.
I am a big football fan and simply watch all matches of my favorite club. For me it is a kind of therapy after a long working day.
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