I've seen this in a couple of other games, and it makes a ton of sense to me. Picture this:
A healer is targeting a tank so they can heal that tank. But they also want to use some attack moves 'cos they have the opportunity, but they don't want to lose track of the tank in the meantime. Could it be possible to allow people to check a little box in Preferences or something that allows them to target their target's target? So the healer could, for example, hit the target the tank is focused on.
It doesn't have to be just healers, either. DPS could do this too - focus on the tank, so they're only taking down the targets the tank is focused on. I've played in some really fun groups that made great use of this. And if it includes the companion, then healers could send their companions in, heal them up, and make some great attacks without having to re-target all the time.
So to sum up: a feature that allows a player can target a character and either heal/buff that character or use attacks to hit that character's target, without having to re-target.