(which is another weird thing about this game, I don't seem to find a way to turn on targeting through someone else.)
In key bindings, there's a binding for "acquire target's target". IIRC it's bound to alt-t by default.
Not quite the same thing as Rosielink meant. In other games it's possible to target through another target.
For example, you could be a healer and by targetting a mob, your heals would go to the mobs target (ideally the tank). This allows you to heal the tank, whilst DPSing the mob, without switching target.
Or, I am used to having a group assist target (raid assist or RAT). This player is normally a DPS character who decides upon which mob the DPS will be focussed. In other games this is easy, by targetting the RAT, one's DPS will automatically be redirected to the chosen target.
In this game we do seem to focus DPS (at least in the good groups I've seen), but it's more by consensus and watching what everyone else is doing, rather than by having a designated player and a game mechanic to achieve it.