Why have a salary Cap?

Almost every sport in America has a Salary Cap. The most logical reasoning for having a salary cap is so that the large market teams cannot just pay players hundreds of millions of dollars to come play for their team and only a few teams dominate the sport. Salary Caps exist so that small market teams without a lot of money backing them up have a fighting chance in getting the big name players in each sport like Peyton Manning for Football. He was recently a free agent and was highly sough after by most teams in the NFL. He ended up in Denver which has a fairly large market, but smaller teams such as Jacksonville had a fighting chance to get him because they could offer around the same salary to him as the bigger market teams. The salary cap creates competition among teams.