Seen as said mainly on new build flats, I thought it may have been to stop tube theft, I normally NSC them for clipping, and normally not sleeving/sealing
Customers, they go up and branch off to each flat. I only noticed because they have teed off the down pipe to a 90 elbow then into each flat.
the meters are to the left and right of the one in the picture.
Its all down to the poxy architect who can't design a building properly so wants the pipes to be covered/disguised to look like a rainwater down pipe. Don't you just love it.
The volume of the pipe needs to be calculated to see if it needs a cent high and low, if volume is under 0.01m3 then it won't require vents, still not right as the pipes can't be clipped inside