6.D.12. After the completion of a rally, players may override a partner’s "out" line call, and officiating team's line call or an opponent’s “in” call to their own disadvantage.

6.D.12. After the completion of a rally, players may overrule a partner's line call, an officiating team's line call, or an opponent's "in" call to their own disadvantage.

Officials are used for tournament play to make unbiased and more accurate calls than players. A player moving on the court will not have a better view of a ball than a stationary official (line judge or referee) or an official reviewing a video replay in slow motion.  Therefore, players should not be allowed to override line calls made by the officiating team. Allowing a player to override an accurate call made by an official is not fair play and can result in players influencing the results of a game/match. It also defeats the purpose of the officiating team.

A line judge makes an "out" call on a ball that lands close to the line. Team A loses the rally but pleads to Team B that the ball was "in." To avoid argument with Team A or to appear gracious, Team B concedes the rally so Team A doesn't lose the rally.