Juule, to help you 'get it' I'll explain why I have one... My DH runs his own international business, he never works less than 60 hours per week - plus commute to London. He works abroad 3 out of every 6 weeks - often away for 12-14 days, home for 2-3 and off again.
I only have 2 DC but my DS (4) has Severe SN (inc CP, metabolic disorder, health problems - heart valve, arterial tortuosity etc) so he has to have physio everyday and is quite physically and emotionally demanding - he has an 80% chance of having a brain bleed before adulthood due to a rare artery condidtion. So I may not have 6 DC, but DS = at least 2 DC!!
For me an AP means I can spend some quality time with each of my children, but mostly so I can give my DD (9) some proper one to one attention whilst DS is being cared for and kept safe. It stops her feeling so resentful and sad.
It also means an extra pair of hands at bath/bed/brakfast which improves my quality of life no-end and stops me feeling so resentful (towards my meglomaniac DH) and sad.
And, TBH if you could get a friendly, lovely young girl to play brilliantly with your DC, help you load the dishwasher and fold all the clean laundry, wouldn't you?