Because stuff happens when you least expect it...what we consider low risk can still have consequences if something happens eg a child knocked over crossing the road or falling off a swing. Thats why having someone outside of the situation who can contact parents or more help whilst you deal with the situation in front of you.
It can also be something happens the other way around, that a family needs to get in touch because of an incident/illness/incident at home and needs the girl home asap. The home contact works both ways. They have the contact number who phones you so you know its something important not just a parent asking if their daughter is having fun!