531D via Phoenix Shopping Centre - Transperth Oddities 

Before the Mandurah Line opened, route 153 used to terminate at this stop on Coleville Crescent and started outside Seton Catholic College on Winterfold Road running through Coobellup roughly hourly very short route but not unusual for that time (considered an oddity now).

I agree that it seems quite pointless and doesn't seem to be used very often - a 531 was always tedious enough without having to make that infernal loop. So at this point, I'd guess it's a relic of a time when Beeliar Village Shopping Centre was non-existent, and Cockburn Gateway was predominantly a sandpit with a handful of shops and a cute little bus station. The old 153 (Coolbellup/Samson (Seton College) to Phoenix via everywhere) used to terminate there on Coleville Crescent. The 531's predecessor, the 135, as well as the 133 (which was the 533 until they just turned it into a deviating 531, which is all it really is) both visited the shops there too. More recently, the 522 terminated at Coleville Crescent after it's marathon trip mostly around nowhere from Cockburn, with selected trips extending to/from Port Pirie Street, Bibra Lake. In the end, it was only those trips that survived, and now the service remains as a "school" special for the sheltered workshop, as another commenter indicated.

Funny enough it's just the Cockburn bound not serving the normal stop at that deviation turn-in while the Fremantle bound is serving its stop before the deviation turn-in, and possibly again after turning back out lol

I am pretty sure the 532 also has a similar deviation, however it terminates at the phoenix shopping centre