1930's maps - nur für den dienstgebrauch

[bananamafia]

Hey everyone, found a few 1938 maps of various places in the UK, they are in german and marked "nur für den dienstgebrauch!" i think this basically translates to "only for internal use"
and wondered why this is so?
would this mean that they were almost "classified" or was there some kind of restriction of use? or were all maps produced in germany at this time marked similarly?

This is way out of my comfort zone so would appreciate any pointers!
kind regards
Ed