Halliday & Matthiessen (1999: 115-6):
We saw that quoted offers may be realised by modulated indicative clauses. This would seem to be an exception to the generalisation that indicative clauses realise propositions. However, it is a principled one: the type of indicative clause involved is precisely the type that lies closer in the interpersonal clause grammar to imperative clauses — modulated indicative clauses, i.e. those with an imperative modality. As can be expected, this is then also a possible realisation of commands; it is a metaphorical strategy for expanding the meaning potential, typically to vary the tenor between speaker and listener.