Halliday & Matthiessen (2014: 195):
The relationship [between the semantic categories of statement, question, offer and command on the one hand and the grammatical categories of the mood system on the other] is a rather complex one. For statements and questions there is a clear pattern of congruence: typically, a statement is realised as declarative and a question as interrogative – but at the same time in both instances there are alternative realisations.
For offers and commands the picture is even less determinate. A command is usually cited, in grammatical examples, as imperative, but it is just as likely to be a modulated interrogative or declarative, as in Will you be quiet?, You must keep quiet!; while for offers there is no distinct mood category at all, just a special interrogative form shall I ...?, shall we ...?, which again is simply one possible realisation among many.