The Levites' cities belong to them forever. The Jewish People are forbidden to take advantage of each other by lending or borrowing with interest. The mitzva of shemitah commands the Jewish People to stop working their fields every seventh year and promises that, miraculously, Hashem will provide for their needs. When the level of the Jewish People's trust in Hashem was low, however, then the fields yielded, in actual terms, three times the amount of a normal year. The first way was through a hidden miracle, the second through an open miracle.
Man is the creature who is designed specifically to have freedom of choice.
Open miracles are so compelling that they limit Man's freedom of choice. The Vilna Gaon replied that Hashem's lot is the Jewish People. He would like us to be on a higher level, but nevertheless He is content with us at whatever level He finds us. One of the effects of the mitzva of shemitah was to plant in the hearts of the Jewish People the idea that Hashem, for all His Transcendence, nevertheless supervises every last detail of this world. Shemitah teaches us that Hashem provides for our needs even though we are seemingly but a small dot in intergalactic space.