Question

why should walls be well plastered and made damp proof

Answer 1

so that they can't be affect by rain

Answer 2

Houses and buildings are the main shelter of the modern day's human beings.

And walls are the most important component of a house or building which protects the indoor part of that house from the natural elements and other hazards.

That's why we have to make well plastered walls to make them well resistant against the natural elements like snowfall,rainfall,heat etc.

And we have to make walls damp proof because damp can easily harm the inner structure of a building or a house , that's why we have to prevent the harmful damp.