Post office directories are the equivalent of City Directories. These are a terrific resource for following your ancestors between census periods. Not everyone was recorded. Since there was a fee to be included, many of those included had some degree of stature - clergy, educators, doctors, professionals, merchants, etc.
A recent addition to the website are the Post Office Directory Maps. Four hundred new street maps of Scottish towns held within Post Office Directories have been digitized and uploaded to the maps website. These are excellent resources for family and local history. These are quite detailed, providing street names, location of public buildings such as churches, shops and schools, as well as railways, cemeteries and open green space.