Your kid might be the next Tiger Woods or something. "<insert world famous athlete/movie star/musician here>'s mom/dad"
"Financial investment". We have a family friend who raised 6 kids on a working class wage, they're now all doctors/engineers/dentists/etc and they just bought their parents a $900k house, go on holidays with their kids and grandkids all the time, etc. Nice :P
Make contacts/connections. Think Game of Thrones "family alliances" kind of thing. In the above example, all of these successful kids have successful husbands and wives who themselves have successful family. You know, your son in law's father is a town mayor, your daughter in law's sister owns a company, etc. These kind of connections are seriously useful.
(Usually in poorer countries) Can help work on the family farm, sewing, cleaning, fetching water, etc (basically cheap labor)
(In many cultures) Children are expected to take care of their parents when they are old, live with them, etc
Fecundism - your children are likely to follow your political/ideological leanings (e.g. if you are very pro-gun rights, or Muslim, or very pro-gay marriage, and so on, your children are likely to be so too), so your ideas will have more weight in the future. For example, Ultra-orthodox in Israel have been accused of having massive birth rates to essentially push out Arabs through sheer numbers.
Economies of scale. Much easier to take care of your 7th kid (when you already have 6) than when you go from none to zero.
Extras:
They can fit through the bathroom window when you lose your keys. (credit u/fruscola)
If people don't have kids the species dies. I just look at it as doing my part. (credit u/baby_monitor1)
Perhaps this is more philosophical, but who really wants to be the first person in their direct line of ancestry to fail to procreate? (credit u/Vincent_Grayson)
Well, I need somebody to make sure the nursing home isn't mistreating me. (credit u/throwawayfortoday121)