Because it's a fancy number. There are no rules. Dnb can be done from as low as 150bpm to as high as 180bpm. Less than the first it becomes more dubstep/electro/garage thing. More than the second it becomes Hardcore/breakcore/IDM.
at the end of day it is the song that defines the swing, not the BPM, however I do like producing at 172 when trying to do write rollers, but that's just personal preference more than anything
i do it because certain things just sound better to me a bit slower idk all my liquids at 172 all my tech and stuff is 174 176 but it could just be because i'm stoned who knows :O
172 definitely seems to be the new 174. I find a lotta new neuro tracks are coming out at 172 recently. Most DJ's seem to play them faster though anyway.