וידבר ה’ אל משה בהר סיני לאמר…(בהר כה:א)

Rashi asks why it says that the mitzvah of shmitta was said at Har Sinai, when all the mitzvahs were said at Har Sinai?

The mitzvah of shmitta shows us that just like shmitta benefits a person since the following year the field produces better, that is the reason why the Torah writes about the mitzvah of shmittah “שבת לשם.” A Yid should fulfill the mitzvah of shmittah because Hashem commanded us to do so, and not because of the field’s benefits.  The same is true of all the mitzvahs given on Har Sinai.  A Yid shouldn’t obey them becasue of his personal interests, he should fulfill them because Hashem commanded so.