Question

salt is used as preservative for some food items explain why

Answer 1

Because it is the antioxidant.
due to this quality when it is put in the food items such as pickle it prevents them

Answer 2

 Salt absorbs water and removes bacteria from the food.Since the bacteria does not multiply (due to the salt water) it acts as a preservative So, basically salt absorbs water from food , making the environment too dry to support the growth of harmful bacteria and mold.

hope it helps :)