What is the purpose of tire cross-grooves and tread siping on commercial heavy vehicle tires?

1. To channel water, slush, and mud away from under the tire footprint to maintain direct contact with the road surface
2. To increase the total rubber area touching the road and so improve dry grip
3. To let air circulate through the tyre and keep the casing cool at speed
4. To allow the tyre to be re-cut deeper once the original tread wears away