What is this called (Dash seal???) and do I really need it?

Well, the factory put them on there for a reason, they wouldn't have gone to the bother and expense otherwise.
My thought is that they installed them to prevent squeaks and rattles like mentioned earlier.
It won't be the end of the world if you leave them off, but I would find a pair and install them.
There should be lots around that you could get for cheap or even free, just start a parts wanted thread on here.