You can appoint a team captain for each match. When the team's total routine is calculated, all the individual players' experience levels are added. After this, a figure representing the team captain's level of experience and leadership abilities are added. This is the team captain's only function, although it's quite an important one, as he may prevent your team from getting confused due to a strange choice of formation, or getting the heebie-jeebies during cup matches or qualifiers!
The team captain has to be in the starting line up. If you haven't appointed a team captain, the players will do so by drawing lots just before the game.