limit unglazed portion to just the ring - the rest of the underside glazed, hence stronger, easier to clean, etc.
limit contact surface - easier to make flat (so the pot doesn't rock) and easier to make smooth (so the pot doesn't scratch surfaces)
provides a place for fingers to catch - easier to handle
allows for precise thickness - better balance, lighter, closer match to the rest of the pot, etc.
cut away the least conditioned part of the pot, the layer against the wheel head (or bat) is most likely to start cracking (per above)
increase durability - ring provides mass and structure at the base, where impacts are likely
takes time to cut foot rings
takes more time to glaze
plates are a special case, bein' flat an' all...
We've had a week of rain and wind here!