Buying a house is a significant investment and you need to know if there are any potential problems before you commit.
A visual survey will tell you what defects there are and the likely cost of repair
There may also be the need for a specific problem to be investigated, i.e. when a mortgage valour spots movement or cracking and requires a structural report. This survey is limited to the specific area (unlike a general survey) and can be done quicker and cheaper than a general survey.
I also carry out surveys of barns for conversion to domestic dwellings as many local authorities require one to be submitted as part of the planning application.