PM is often involved far more in the budget and over all co-ordination of the production than the SM - whilst the SM (or PSM) is responsible for the day to day running of things, the PM involves themselves in the longer term items - permits, organising site inspections, getting permits, ensuring OH&S policies are followed etc etc etc etc. Basically, a show that involves a PM takes some of the load off of the producer/director and some of the load off of the SM. Generally the job is death by paperwork and meetings. Been there, Done that... No thanks.