Halliday & Matthiessen (2004: 419):
Verbal conjunctions are derived from the imperative or from the present/active or past passive participle + (optionally) that: provided (that), seeing (that/how), suppose/supposing (that), granted (that), say (that). In origin these are projections; their function as expanding conjunction reflects the semantic overlap between expansion and projection in the realm of ‘irrealis’: ‘let us think/say that’ = ‘if …’, as in say they can’t mend it, shall I just throw it away?.