Yeah defend works the opposite to what you might think.
You don't assign a destroyer to protect a carrier, you assign a carrier to protect a destroyer.
It might be easier if you think of the order as "Defend Arse." As they will endeavour to defend the targets rear end but not it's face.
EDIT: Oh my reading comprehension is poor.
Yeah it's basically just an attempt to have a ships somewhat nearby to 'defend' the area with no real preference as it who is in front or behind. I'll often assign several defence targets at a battle start with some ahead of the others for my gunboats, and the rear most ones for support ships like carriers, to create a very elastic fleet formation.