Not having a copy of the Act to hand, I can't be sure, but I believe it set out the mechanism by which an Air Force would be formed - i.e. by an Order in Council. A draft Order had been prepared by 13 December 1917.
If Jan Smuts' report was effectively the conception of an independent Air Force, the Air Force Act might be regarded as the ante-natal class preparing for its birth on 1 April.
It took me a long time to realise that the choice of 1 April may represent the start of the Civil Service's financial year, and thus a new budget line.