The National Football League (NFL) began in 1920, but the New York Football Giants did not join the league until 1925. They were one of the first large city teams and played in the 14-year-old Polo Grounds with a seating capacity of 70,000.
Not that the club needed that much space for patrons. At the time, all any sports fan cared about was major league baseball and college football.
Professional sports develop enormous benefits from star athletes. They became all of our heroes. And the more heroes a team (or sport) employs, the more folks want to buy a ticket and see them play in a live environment.