Shopkeepers selling ice blocks have to prevent the ice blocks from melting so they cover ice blocks with the jute sacks or any other such material since Jute sack is an insulator (Bad conductor of heat- does not allow heat to pass through it), so the jute sack prevents the ice from melting as

a) it does not allow the heat from outside to pass through it and enter into ice.

b) Also, jute sack prevents the cold to pass through it.