Planes, Trains and Automobiles

1. Basically, to “pump” the water away and give direct contact between the tyre and the road. Without tread you’d possibly get aquaplaning or what might be a “wedge film thrust bearing” in the wet with very small reaction forces between the road and the tyre meaning you might not be able to steer or brake.
2. Conservation of angular momentum. Main Rotor speeds up or slows down would mean the body of the helicopter would need to rotate to conserve angular momentum. Instead the tail rotor provides an external moment.
3. diesel has a lower “flash point” than petrol enabling it to combust under compression without a spark (although sometimes diesel engines need some help when initially started via “glow plugs” or whatever).
4. overall efficiency
5. Well it shows the tyres have actually hit the ground. Some of the energy has gone from heat into smoke so the plane is slowing down which is a good thing of you want to get off. The indication is of dynamic friction coming into play with some sliding of the tyre which is not quite as good as static friction (or rolling in this case).