Some context, for people not located in the UK - A full British postcode typically aims to cover around 15 buildings (sometime a single building, sometimes a street of 50 houses). This is in contrast to many other postal code systems which cover relatively broad areas).

Or to put it another way -

UK - 2,643,732 codes, 1 code per 25 people,

USA - 41,700 codes, 1 code per 8000 people,

Germany - 8,200 codes, 1 code per 10000 people,

This means that post-codes are often used as a proxy for an exact location - e.g. if I am going to visit a relative, I can enter their postcode into my sat-nav, and be confident that most of the time I'll get to within a hundred meters of their location.

This doesn't work so well in rural area or on large estates where the access point may different from the location, leading to places sometimes advertising a different postcode to put into your sat-nav (e.g. of where the site entrance is) to that of the location itself.