Placing religion aside – a child is your genetic legacy, and the only way that you will truly live on once you’ve passed away. True, friends and other family may remember you fondly, but leaving behind a child (especially one of the same gender) is a quasi-rebirth of sorts that reaffirms your existence as long as they are around. Even from an agnostic/atheist view, I think that’s a pretty significant reason.
I love hearing from older people I meet that they knew my grandfather or know my father and how much I remind this perosn of either of them. It may sound cliche to say that they see a little bit of your parent/grandparent in you, but I find it to be very true.
People do inherit bits and pieces of their forebearers and that makes those anscestors continue to have a very real presence in our physical world.