It's quite funny that there are people who think parent and child bays are about 'being lazy'.
As has already been stated, try getting a baby out of a modern car seat without having the door wide open. Also the need for a safe path through the car park without having to run the gauntlet of cars trying to park is an obvious one with a child in tow.
Many times I've parked in regular spaces because all the parent and child ones are full and it does wind me up when someone walks back to their precious Q5, or whatever car they deem too superior to park with the muggles, without a kid in sight.
I've also had the situation of being on my own with my, at the time, 14 month old. Trolley full of shopping and my car pinned in between 2 SUVs with 'just about enough room to squeeze in' on both sides. Not fun but more importantly, not safe for anyone.