What is a fundamental reason for deck registration in tournaments?

The fundamental reason for deck registration in tournaments is to ensure the integrity of the tournament by preventing cheating. By requiring players to record their decks, tournament organizers can verify that players are using the same deck throughout the event, which helps maintain fairness in competition. This process allows judges to check for deck legality and consistency, making it more challenging for players to engage in deceptive practices such as switching cards in and out of their deck between matches.

This practice not only aids in maintaining the fairness of the competition but also helps in upholding the spirit of the game, ensuring that all players are competing under the same rules and conditions. By having a clear, official record of each player's deck, tournament staff can quickly and effectively resolve any disputes regarding card legality or composition during the event.