I agree Jen
Today photographs are two a penny, we take them and delete them at a whim. Those of us who are "older" remember when photography involved buying a film, taking the pictures, then waiting to get the film developed, only to find you had cut off half of Uncle Harry on the right and Auntie Maud on the left had blinked and had her eyes closed and little Johnnie was picking his nose. Even then it was commonplace - most families had an "instamatic" or before that a Brownie box.
But, going back a bit further, photos were a rarity and a luxury. I am often bemused on here when people ask generally "I was hoping someone would have a photo of my xxxg-grandparent". The chances are there was never a photo taken of him/her, or if there was it would have been a one-off for a special occasion and thus if it survived would likely be with the family anyway.
So, for a family for whom having a photograph taken WAS a possibility (cost-wise), how easy it must have been to either have thought "we will get round to it sometime" but never quite get round to it, or alternatively it was never quite a priority cost-wise and so got put off.
Then, one of your children dies. You are faced with that awful knowledge that you will never ever see them again.
I can see that the idea of a PM photo could be something that would be considered , so you have a picture of them, which otherwise you would never have.
The idea of it seems awful to us now, but I am sure any family facing the trauma of losing a child nowadays will already have a picture of them, probably a whole lot of pictures throughout their short life.
Likewise for someone losing a husband or a brother or adult rellie.
And has just been said by Cazza, in circumstances nowadays when the baby dies at birth before there has been any chance to take photos, then photos of the deceased baby can be a great comfort and memory.