A front escort usually serves as a proactive, early warning resource to notify the trucker of any oncoming traffic or potential hazards ahead.
A rear escort serves as the buffer between other traffic and the truck & load, often occupying offset or multiple lanes behind the load to protect traffic while the truck and trailer make a turn or prepare to make lane changes due to lane merges/closures, an obstruction in the road ahead, or shoulder-parked vehicles. The above photo is courtesy of Adobe photos.
When the load is over height restrictions, most states require a pilot car, called "Height Pole Car", in front of the tractor-trailer. This type of car has a telescopic pole attached at the front of the pilot car which is adjustable to match the height for that partticular load in order to clear bridges, traffic signals, power lines or other overhead obstructions.