it buffers you from dark billing patterns of all companies except apple
But your spouse has to know you (or which family member) bought it and click on their name in “family sharing” to get it for free. Else spouse will pay for it again.
It doesn't quite work like that. When someone in your family goes to hit the purchase button, it pops up a window saying that someone else has already purchased it. I'm not sure why you have to hit the button first, maybe for some measure of privacy from your family members?