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)
You're usually not allowed to bat on yourself for fairly obvious reasons.
On a related note, a friend who's a sports better always placed the biggest bets against his own teams. Partly because he trusts his judgement better when not clouded by the hope that his team will win, partly because if he looses the bet, hey, his favorite team just won the game against the odds!
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)