Did you know that if you add your $10 to the DPS’ $100 then you get $110, which is more than $100.
There’s only one damage number that matters and it’s not the output of the DPS. It’s the sum total damage of the group. The boss does not care who the damage came from. Damage is damage and more of it will always kill faster. Faster fights are safer fights. Doing damage is useful no matter how small it is compared to the DPS’ numbers. If there was an NPC at the start of a dungeon that you could talk to to give your DPS a free 10% boost, would you do it? Of course you would because it’s a free boost. Healers doing damage when no healing is required is exactly the same.