Building sites have a different set of operating conditions, working amongst various trades in the final days of a construction project. Site compliance is critical, with OH&S considerations very important before the job even starts.
Builders cleans are different in that they need to touch all surfaces of the property, from windows to carpets, and then getting at the dust that accumulates on ledges and inside cupboards.
External areas can sometimes need pressure cleaning and the yards will often need rubbish removal and a tidy up. Ultimately, the client who will take possession of the property on project completion just wants it to be ready for use.