[1] A vehicle is not supposed to pass a timepoint until the schedule time has arrived.
At minimum, it allows regular passengers to estimate when a bus would get to a stop before or after a timepoint.
The creation of digital schedules, and real-time geo-located apps has somewhat complicated the situation for agencies operating with timepoints.
Agencies may interpolate between their timepoints, but this may not reflect a realistic expectation of when a vehicle should arrive, and the GTFS specification cautions against this practice.
[3] A bus bulb, where buses stop in a traffic lane, is also not suitable for a timing point.