The salve wasn't technically necessary, but Maedhros recognized Aragorn felt the need to do something, anything. And it was soothing, he recognized Elrond's mix in the effect.

That proved his theory. He'd been there that day too. But his efforts had been in vain and he'd yet to forgive himself his failure. If not for Elrond's own sons... "I am grateful you and your mother were not slain."

Elrond had always been kind, even when he had less reason to be.