Your children will only be this age once. The way your daughter climbs into your lap without asking, the specific way your son laughs at something only he finds funny — these are the things you'll want to remember, and the things you won't think to write down.
Most of us have thousands of photos on our phones, but very few of them are of us. We're usually the one holding the camera, not in the moment ourselves. A family session is one of the only times someone else does that job for you.