It's to give time for players and their team to get together and discuss What has been going on in the game so far. To solve a problem or to Pep talk the players. Also it gives player a short break from playing.
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I'm certianly no expert in the game, and there may be a more accurate historical reason, but logic suggests that since football is such a physically intense game, often lasting 3-4 hours, the halftime allows a brief rest for the players.
for the players to rest as it is a really tiring game and for a team talk and game plans!