What is the significance of the words "Shabbos la'Hashem"?
Ramban: The Torah writes "Shabbos la'Hashem" in connection with Shabbos Bereishis, because Hashem Kevayachol, rested on it, 3 which is why we may do no Melachah on it. And it uses the same expression regarding the Shemitah because it is the seventh (the Shabbos) of the cycle of years.
Seforno: It implies that, since one is not occupied with tilling the land, one should spend the year in the service of Hashem, 4 to a certain degree even the farmers, who are not on such a high level. 5
Moshav Zekenim citing Ramban: Shemitah is testimony that the world was created for 6000 years, and the seventh millennia will be Shabbos and eternal rest. One who denies Shemitah is like one who denies Creation and the world to come, like it says about Shabbos. This is why the Torah is so stringent about Shemitah; for not observing it, Yisrael are exiled.
Rashi: Where the Torah uses the same expression (in Shemos 20:10). See Ramban's objection to this explanation.
Moshav Zekenim: Resting the land improves it (it will produce a better harvest in the ensuing years).
Seforno: Just as one is supposed to do on Shabbos, where the Torah uses the same expression (in Shemos 20:10).
See Seforno on Pasuk 4.