Halliday & Matthiessen (2014: 168):
Proposals are typically combined with tones 1 [fall] and 3 [level~low rising]. Imperative clauses, functioning as commands, typically favour tone 1, as also do modulated declaratives; but a mild command, such as a request, and also a negative command, often comes with tone 3, which has the effect of leaving the decision to the listener. For the same reason offers are commonly associated with tone 3.