Traditionally, most buildings were - and still are - constructed under the management of experienced and professionally qualified building contractors.
Over recent years there has been a tendency for smaller building works to be carried out by 'self builders' acting as their own project managers and contractors. However, since many of them have found this process too onerous and worrying, many have turned to independent Project Managers to look after the project for them. They will look after the surveying, assess what materials are required, decide what works to place with sub-contractors, organise contracts and place orders on your behalf as the building owner. Effectively they can act as the Building Contractors manager, but working for you as your own contractor.
To save money an increasing number of people are also taking on some of the 'sweat', or 'grunt' tasks that (they think) do not require specialist skills or experience themselves. Generally this takes them much longer than it would to hire a professional 'subbie'. While they don't have to pay them, they will incur the extra costs related to the additional time required.