1 - they eventually start their own career and make money for themselves, often much more than the parents
2 - they're able bodied when the parents are elderly and need able bodied help
3 - they offer grandkids for the parents to look after and cherish
I could list more but that's just a few material dividends
1. That depends on a lot of factors now days millenials and gen z are poor af can't afford a house and increasingly stay longer at home and leech off their parents. This is statistics
You know what guarantees you will have more money not spending it on kids in the first place. I could put that money into s&p 500 or some real estate rental and it would get me a guaranteed return in 20 years don't need to hope on your children to do well.
2. Most kids are obese and have far more health problems then the previous generation. If you are elderly and need help just hire a young caretaker does the same thing. Kids cost more.
3. If you don't have a purpose or some drive to wake up to every day other than grandchildren you are boring ego driven old fart anyway. For example why do they have to be your grandkids? There are billions of them now just dedicate your life to all grandkids not just "my" grandkids.