Having already taught us that all fields revert to their original owners in the Yovel (See Pasuk 10), why does the Torah repeat it here?
Rashi: To include a case where the owner's son redeems the field that his father sold - which also reverts to his father in the Yovel. 1
Seforno: The Torah is teaching us here that the owner is permitted to establish ownership over the field - to build houses, dove-cots and sheep-pens, as long as does not work the land or perform acts that guard the fruit.
See Sifsei Chachamim.