What they are genuinely best placed to do
Anything that requires the school's own records or the school's own voice. Course selection and whether a schedule actually meets graduation requirements. Whether a class your child wants is compatible with the rest of their timetable. The school report and counselor letter that accompany applications, which no outside party can write.
They also know your specific school in a way nobody else does — which teachers write strong letters, how the school's scale is read by universities, what happened to students in similar positions in previous years. That is local knowledge and it is valuable.