PVA bags are used mainly by carp fishermen, However I have used them for beach casting.

The PVA bag dissolves on contact with water and releases whatever it contains. I make a bread, old bait and pilchard oil mash. I make it very dry so it won’t effect the bag. Course anglers use them for boillies and bread to ground bait.
If I am beach casting I then lower the weight into the PVA bag and seal it, then cast it out.
The idea is this produces a strong scent trail and attracts fish to your bait.