Can be useful for electronics enclosures, aerials, conduit.
Plumbing pipe is designed to with stand pressure, but not necessarily UV. Electrical conduit for outside use should be more UV / weatherproof.
Maiana uses some water pipe for the case, but it seems to be an American size which is harder to get in UK (although I did find one supplier).
Plastic pipe in UK seems to be speced by OD.
e.g., I measured some 15mm nom. PEX. Pipe marked 15mm x 1.5mm. Measured OD = 15.0mm, ID = 11.5mm. Wall thickness measured more like 1.75mm.
Pipe insert / stiffener, OD ranges from 11.5mm at tip to 11.7mm at flange end. Flange 15.0mm.