Tournament attacks have the same rules as basic raids, in which you attack someone & their base with your troops (heroes) to take stuff from them (cups, food, iron etc.) If your heroes die & your opponent’s heroes die, you didn’t “win” since there is no one on your team to take the loot back to your base. There is a discussion about it somewhere on the forum. (e.g here:)
Some people think it’s dumb, some think these rules make sense 