You really want to document and cherish this time, maybe it is to celebrate your body and what it is doing - growing an actual human! It's a pretty big deal! Or, maybe you are a busy parent already and time is flying. Each moment with your baby/children is fleeting and you want these special memories captured to remember this time.
Besides - as parents, one of you is usually behind the camera, and it's hard to find any with all of you in. Looking back over images from the past you are often missing from those moments and memories. Which is such a shame! Like, most people, you would probably love to be able to look back through photos with your children when they are grown up, and share what life was like. Well, don't worry, I have your back!