Why should lowest branch connection be at least 450 mm from invert?

[Shevek]

UK drainage regs say: "for single dwellings up to three storeys high, the distance between the centre line of the lowest branch connection and the invert of the drainshould be at least 450mm."

What's the technical reason for this?

 

[pmc]

prevents back pressure. which is a pressure build up of compressed air at the foot of the stack which could cause trap seals to fail