See Chess Clock

People often wonder why there are time controls in chess, seeing as chess players are naturally not going to play as well as normal when they have, let's say, 5 minutes left to make the next 10 moves or they will forfeit the game.

Bobby Fischer used to tell a story about the old days of chess, before chess clocks were invented, when players in a tournament were only limited by etiquette as to the time they took to think about their next move. Paul Morphy, the most brilliant player of his era, and a very fast mover on the chessboard, was playing Louis Paulsen, a very good, but very slow player. On one particular move his opponent sat fixedly in his chair, gazing at the board, without twitching, for something like two hours, with Morphy growing more and more visibly impatient, until finally he leaned forward and said, "Excuse me, but why don't you make a move?" Upon which Paulsen jerked slightly, and with a look of surprise said "Oh, is it really my move?"

And that is why we have time controls in chess.