In both the NFL and NCAA, there are official capacities that the captain must fulfill. Only one designated captain (at a time) is permitted to communicate the teams official decision of a penalty (to accept or decline) to the referee, and there are rules about how you can change captains.
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
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
Note: A captain on the field has no authority to request a change of fellow team captain when that captain remains on the field
Rule 1, Section 1, Article 5. Each team shall designate to the referee not
more than four players as its field captain(s). One player at a time
shall speak for his team in all dealings with the officials.
Rule 3, Section 1, Article 1. b) Three minutes before the scheduled
starting time, the referee shall toss a coin at midfield in the
presence of not more than four field captains from each team and
another game official, first designating the field captain of the
visiting team to call the coin toss. Before the second half, the
referee will obtain the teams’ second-half options
Rule 3, Section 3, Article 7, g) The field captain must exercise his penalty option before he or a teammate
consults with his coach on a sideline during a timeout.