It's usually seen as a sort of parallel form to "May the king live long!" or "Let the king live long!" and traditional grammar often explained it as "subjunctive" - which of course would be true in other languages but has no validity as a separate linguistic form in English. (But then neither does the term imperative and we go on using it
) It's also found in expressions like :
Come what may...
God save the Queen!
Heaven forbid that...
Be that as it may...
So be it then!
Suffice it to say that...
It is a sort of "imperative" - in both cases the base form of the verb is used to express what the speaker wants to happen. Which for me, points to the real explanation : that the base form of the verb is used to express strong volition imposed by the speaker - ie a desire that something currently unrealised becomes true (the dog is not yet sitting but I want him to, so I say
Sit! / I want you to be content with the general picture so I say
Suffice it to say! Notice how all the non-command examples are formulaic, set phrases though - it's very limited in its occurrence.