Converting from UK Postcodes to/from lat/lon or OSGB coordinates
Smoothing boundary lines or routes produced by GPS devices to display on a map. A typical file from a GPS device will contain 1,000's of points. A mathemtical process is used to "smooth" the routes to a few hundred points loosing only a few metres of accuracy
Displaying Locations (e.g. a shop) or lines (e.g. a boundary or driving route) on Google, Bing, or OS Openspace maps.
Creating Points of Interest files (e.g. TomTom)
Adding elevation data to routes (from satellite radar mapping) in order to create an elevation vs distance graph. This is much more accurate than using elevation data from GPS devices which is often highly inaccurate. Note that GPS elevation varies significantly from Ordnance Survey elevation - by around 40 metres (120 feet) - as they use different models of the Earth's surface. Mean sea level varies between the UK's east and west coasts.
Screenscraping, e.g. of Wikipedia, to get geo-data.