A pickup by appointment request means specifying the time and date that you wish your shipment to be picked up to the carrier.
These requests are common in FTL (full truckload) freight shipping.
In LTL (less-than-truckload) shipping pickup appointments are not performed due to the fact that trucks leave their terminals pre-loaded with cargo out for delivery that day. That means that it is impossible to know how much time it will take any single truck to finish their deliveries or prior pickups before arriving to any given location.
Delays happen for numerous reasons, including but not limited to congestion, delays at individual pickup/delivery locations, weather conditions, truck breakdowns, etc.