The short answer
A PMC manages schedule, cost, contractors and quality on your behalf when design and execution are too complex to run alone.
A project management consultant coordinates consultants, tenders, contractors and site progress, and reports on cost and time. They do not usually design; they make the design process deliverable.
For a single house, an architect with a defined supervision scope often covers this. For multi-unit or commercial work, a separate PMC is usually justified by schedule control alone.
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