What is the reason to cover the DTH dish with some covering like plastic bottles, etc.?

This is done to save Dish LNB from water in rain. There are chances that water through Dish LNB may enter the cable and the water may affect the signals and hence, the picture quality. Further water through wire may enter the Set Top Box as well resulting in short circuit.

To avoid this, the most important thing is to have a loop in cable near the Dish. Also, there must be a cap on the cable at the place of joining of cable with LNB to avoid entry of water. If there is proper capping on cable, then covering of LNB is not required.