Say that the difference between winning and losing a final is £50k, and the bookmakers pay out even on a bet that you lose. If you bet £20k on you losing, you change a "nothing if I lose, £50k if I win" game into a "£20k if I lose, £30k if I win" game.
Some players prefer such a game. Placing that bet might even increase their chance of winning, as part of the stress gets removed from the game (you're playing for £10k, not for £50k)
Say that the difference between winning and losing a final is £50k, and the bookmakers pay out even on a bet that you lose. If you bet £20k on you losing, you change a "nothing if I lose, £50k if I win" game into a "£20k if I lose, £30k if I win" game.
Some players prefer such a game. Placing that bet might even increase their chance of winning, as part of the stress gets removed from the game (you're playing for £10k, not for £50k)