Minor league sports teams play a very important function for many sports. These teams are considered feeder or farm teams for the pros. The players get professional development on a team affiliated with an MLB or NHL team. Usually based in smaller cities, the MLB affiliate teams are also a place for contracted players to go for rehabilitation. The big goal of a non-contract player would be to be “called up” to the big leagues.
The NHL, similarly, has affiliate teams in the American Hockey League. They might have a two-way contract with a prospect allowing the player to be sent back and forth between AHL and NHL. Other players can be contracted exclusively to the AHL, although you can bet they are hoping to get noticed and contracted to an NHL franchise.
Major League Soccer in the US also has reserve teams in the United Soccer League.
A major professional sport in the US that does not rely on a farm system is football. The NFL typically drafts its players from the National Collegiate Athletic Association’s Division I teams.