What is the purpose of the 'Lift Axle' (Liftachse / Anfahrhilfe) on a 3-axle or 4-axle heavy truck?

1. To raise the vehicle over railway tracks
2. To fly the truck over traffic congestion
3. To lower tire wear and rolling resistance when driving empty, and to temporarily transfer weight onto the driven axle (Anfahrhilfe) to gain traction on slippery hills
4. To allow the driver to change tires without a jack while driving at full speed