There are various reasons that lead people to consider overnight care. When a loved one needs special attention a caregiver could arrive a few hours before the senior goes to bed or perhaps make dinner or supervise an evening shower or bath. Having someone perform extra duties would conform to the schedule and wishes of the senior.
Some older people have great difficulty sleeping, so it is very helpful to have someone to be there for a midnight snack, scheduled medication or to escort the patient to another room. The senior may be irritated or fearful, needing reassurance and a strong arm to hold onto. Maybe they want to get up for a while and it is better for someone to supervise. Later the caregiver can gently help get the senior back to sleep.
Because some seniors are embarrassed if their family member wants to stay over, they might sit in wetness. It is easier to be cared for by a non-family member for intimate and potentially embarrassing personal care needs, because family history can make these sorts of interactions awkward.