I have two questions about zone hardiness:

1) why does a plant even care how cold it is once the temperature drops
to, say 10 F? Why would a plant be damaged more by -40F than -20F? Is
it the depth of the frost, or something else?

2) In the colder zones, does the snow really protect the plant from
cold, or just from the wind? I would think that if the air temperature
is 5F all day, wouldn't the ground even under the snow also be either
5F or colder? I don't understand how "insulation" can help when
there's no heat source: if you wrap an ice cube in a mitten in a
snowstorm, the ice cube will still be cold.

Thanks.