SWMS - Safe Work Method Statements
A Safe Work Method Statement - also referred to as a SWMS - is primarily used to train your workers in the safe use of an item of equipment or work activity.
The reason your builder or principle contractor is insisting on it, is that they seek WRITTEN proof of that safety training, and a SWMS is the accepted format for that.
The Person conducting a Business or Undertaking "PCBU" must carry out the review of SWMS in consultation with Principal Contractors, contractors, subcontractors, workers and any relevant HSR's at the worksite.
Reference: Code of Practice - Managing Risks on Construction Work - Safe Work Australia (2012)