I was told that the 5m are usually kept clear where the city and/or traffic planners expect a road widening at some point in the future. But in principle, all reasons here are more or less plausible.
Funny anecdote: In the run-up to the planning, I was always told 5m distance from the road and that nothing else is allowed there, they must be kept clear, not even a garbage bin shelter is allowed there. But the neighbor built 4m to the street. I had 4.6m in the application, it was approved without problems. At some point, someone said the 5m are measured from the center of the street. I don’t quite believe that either, but how it really is, I will probably never find out...