In the end...the biggest reason probably is that is gives them and excuse to see you. Not saying you don't see your family a lot but for a lot of people, without kids, they tend to see mom/dad less and less to some extent. If they have grandkids there is that built in "reason" to see them or for them to visit you.
Also, people LOVE giving advice and being involved in other peoples lives (just look on this site with the crazy advice people give) and parents can't wait to tell you "how to do it" and stuff. Not in an evil way...just in a typical "I did this before, I can tell you what to do" way.