I get my Sims to have children purely out of curiosity as to what the offspring would look like from two different Sims and to see how much genetic variation there is with each new child. I usually then get bored with the children once they appear, have a play around with looks in CAS and quickly age them up (sim babies, toddlers and even the children are just too needy!)
With most of my sims who have had children, I ended up missing the times when they were all just single young adults and have started a few new saves with them in that stage again.