It's basically a leadership thing and the designated player for communications on field.
Rule 18 Guidelines for Captains Article 1 * One hour and thirty minutes prior to kickoff: Respective coaches designate the captain(s) a maximum of six per team.
Article 2 Coin toss:
(a) Up to six captains per team can participate in the coin toss ceremony; only one captain from the visiting team (or captain designated by Referee if there is no home team) can declare the choice of coin toss.
(b) The team that won the toss may then have only one captain declare its option.
(c) The team that lost the coin toss may then have only one captain declare its option.
Article 3 Choice on Penalty Option: Only one captain is permitted to indicate the team’s penalty option.
Article 4 Change of Captains:
(a) The coach has prerogative of informing Referee when he wishes to make a change in team captains; or
(b) A captain who is leaving can inform the Referee which player will act as captain in his place when he is substituted for; or
(c) When a captain leaves the game, the incoming substitute is permitted to inform the Referee which player the respective coach has designated as captain.
Note: A captain on the field has no authority to request a change of fellow team captain when that captain remains on the field