She was the original owner of the ruby slippers that brought Dorothy home and which the Wicked Witch of the West (her sister) so coveted for their magical powers. Until Dorothy inadvertantly squashed her, the Wicked Witch of the East was oppressing the munchkins, why is why they were so very grateful to Dorothy for (from their perspective) crushing the witch. (In Oz you can't kill anyone, but being good and flattened is close enough.)