Question

why silver coated is there inside every snack cover or packet

Answer 1

The silver on the inside is usually foil or a metallised polymer film which has metal deposited on one or both sides. The colour on the outside is generally printed on the other side of the foil or un-metallised side of the film.
A lot of packaging (such as coffee bags) are actually 2 or more layers of polymer film, metallised film and foil in combinations to give the desired properties. The outermost layer is printed with a design and the other layers are used to give properties such as air/moisture barrier, pack strength (to stand the pack vertically for example) or for sealing properties (peel seal vs a weld seal).
The silver on the inside is generally the foil/metfilm as it gives high air/moisture barrier properties and keeps things cool and fresh.

Answer 2

The inside of the pouch shows you the innermost layer of silver color but it is not actually the silver one. The outermost layer on the inner side is of polyethylene which helps in pouching or sealing of the plastic lamination. It is the middle layer that is silvery in appearance.

The chemical coated high barrier metalized polyester film thus is silvery in color and acts as a barrier to resist outside environment which consists of many things at once. ...hope it helps you