The idea
Given the blanket smoking ban imposed by the last Government, why are signs needed on every premises saying that smoking is banned. As long as the ban exists, it is obvious that smoking is not allowed inside the premises.
Before the ban, you did not see "No smoking" signs at entrances to churches, schools etc, it was obviousl to everyone that it was not allowed. It is just another example of the nanny state.
Why is it important?
It has involved additional, completely unnecessary, cost of every owner of a public building and adds to the clutter on the street frontage.