Slightly confused

So I had to call my OB office after hours because I thought I was leaking fluid ( it was a false alarm 🙌🏾) but the nurse on call asked me if I was using a doctor or a midwife..I was so confused. Only because every time I go into the office I see who ever is on the schedule for that day. So my question is..what is the difference? Like why would it matter which I prefer if when I go into labor I'll get who is on call for that day/night...someone plz help me lol