Two trains of thought here. Keeping the tissue moist keeps it viable for possible shell closure or repair. There could be some exposed tissue that you really don't want to descicate.

Letting it dry out kills a lot of the bacteria and fungus that is found in the water. This is the route to take if the wound is to be left open to heal by second intention; a decision the vet would make after seeing it.

How did the vet visit go?

Ian