It’s partly a waterproof barrier to help prevent wood rot, and it’s partly there to hold the deck together in case the nails and fasteners fail. Can’t go wrong.
I’d say 9 out of 10 are in pretty good shape. I’m sure they’ll hold up. My biggest issue is there aren’t enough of them! These joists are spaced at 24″, and I need 12″ to 16″ spacing for the composite decking. I’ll be adding in new joists in between the existing ones.