This is because although a declarative statement can state facts given that the speaker is not consciously lyingit can also express something which is not true. The vocative case of nouns can be said to indicate the imperative as well since it does not seek information, but rather a reaction from the one being addressed.
All that being taken into account, we can more clearly see why a statement such as, You are not going to that bar. Dessverre husket hun ikke datoen. The list goes on, and as a matter of fact, all 12 combinations between each of the four functions and their three other counterparts should be theoretically possible.
Imperative sentences express commands which convey the desire of the speaker to make someone, generally the listener, perform an action. Whose team has won the match? Sometimes the alternative contains only a negation: Will they ever stop arguing or not?
They many times feel like involuntary reactions to a situation, yet they can technically be stifled if need be. The imperative sentence expresses inducement, either affirmative or negative. Let us have some tea. No curtain.