Why must I hold back 7% when I buy a strata unit from an owner developer?

Section 88(2) of the Strata Property Act, read with Regulation 5.2, requires the purchaser of a strata lot from a owner developer must retain 7% of the gross purchase price, for a period of 55 days after the strata lot has been conveyed. This is to allow contractors who have not been paid to file a lien. If no-one does, the holdback is paid to the owner developer.