If you work over an AVERAGE of 48 hours based on whatever reference period is being used by your yard, then you do need to book POA lest you rubbish your 26, 17, or whatever week average.
I have often heard it said that “POA is a management tool” - Well, it is and it isn’t. You cannot set out INTENDING to be on POA at a certain time. That’s other work. UNEXPECTED scenarios that keep you stationary with the engine off, but available to move again at a moment’s notice are to be booked as POA.
Eg. M25 accident park-ups, being ■■■■■■ around at RDCs before you’ve even got on a bay (but not whilst on the bay which must be booked as “other work” since you are not in control of your movements during the time the keys are handed in!)
The “management tool” is used correctly by firms such as Royal Mail who won’t let you spend more than 48 hours on duty average per week. If you are agency therefore, you get to pick up a LOT more hours than full timers do - providing you’ve got the “spare hours” on your digicard to enable their management policy to permit you to do such hours. Indeed, picking up a 12 hour shift might well happen because no full timer has enough hours left to do it!
Their 36.25 hour basic week works very nicely to throw 12 hour shifts my way for that very reason! 
RM say they “don’t recognise POA” but since waiting time is fully paid for, and you’ll not get a week of over 48 hours as a full timer there, I’d say the pay is actually quite good based on those limited hours. As an agency going in there, I’ll book POA (on the digi at appropriate moments) regardless, because I might find myself cramming in an extra shift at the end of the week, if I’ve been given less hours than what my card will stand. 
Notice I say “at the end of the week”, because if I did another shift in front of a week at RM, they wouldn’t give me the damned hours in the first place, because it would ‘put me over’ as far as they are concerned.
I may have a very low monthly aggregate, but RM will resist anyone even on a “week one” basis going over 48 hours, but once you’ve done the week, your card has been downloaded, and you’re doing a 15 hour shift that coming weekend? - That’s no one’s business, “post card download” is it were. Thus, I might do 20 hours one week total, 40 hours the following week, then cram a 48 hour week at RM, rounded off with a 15 hour at a supermarket (63 hour week measured Monday-Sunday), but the monthly/reference period average is 41 hours, and the POA will be used only to get the 63 hour week back down below 60 for those firms that start their week on a Monday as opposed to the agency week that starts 00:01 on sunday morning, thus as far as the law is concerned, I’ve actually done hours of 20+40+48+15 over a FOURTH week which is perfectly legit as long as I don’t cram another week in after that last sunday!
Thus, POA is YOUR management policy to get around the differing rules held by different yards, so you can work the hours you want, and not be fobbed off about what’s legal and what is not.
Being on duty 84 hours a week, week in, week out is what I consider to be illegal, and I stand by that judgement. 
If you have a job where you never spend more than 48 hours on duty, week in, week out, then you’ll NEVER Need to use POA - thus there will never be any to book any in the first place. 
If, on the other hand, you have a job contracted 48.15-168.00 hours per week, then you’ll be extensively using POA to a degree that amounts to abuse of the system - because you’re trying to hide in cahoots with your employer those hours that would otherwise take you over the 48 hours on duty average across a reference period. This directly leads to insufficient sleep periods, irregular meals, and being addicted to being at work to such an extent that lives are always going to be at risk - yours and others alike.
If you do manage get a full nights sleep whilst on POA at work - then bully for you. Most drivers don’t, and the sensible laws should be about what apply to most drivers, not just the lucky few with easy going employers. 
The driver is responsible for their hours , losing their licence,being heavily fined, or even imprisoned if found to be falling foul of the law.
Eg. kill a cyclist, and you’ve been on duty 84 hours that week, with 40 hours of it mitigated away as POA - Sorry bud, you’re still going to jail - make no mistake! 