We first learn about the laws of Shmita (the Sabbatical year) in Parshat Mishpatim, Shmot 23:10-11:

Six years shall you sow your land and gather in its produce. And in the seventh, you shall leave it untended and unharvested, and the destitute of your people shall eat, and the wildlife of the field shall eat what is left; so shall you do to your vineyard and to your olive grove.

In the following verse, Shmot 23:12 we read about the observance of Shabbat:

Six days shall you accomplish your activities, and on the seventh day you shall desist, so that your ox and donkey may be content and your maidservant’s son and the sojourner may be refreshed.

In Parshat Behar (Vayikra 25:1-6) the Torah gives us a more in depth explanation of the laws of Shmita.

God spoke to Moshe on Mount Sinai saying:  Speak to B’nai Yisrael and say to them: When you enter the land that I assign to you, the land shall observe a Shabbat of God. Six years you may sow your field and six years you may prune your vineyard and gather in the yield. But in the seventh year the land shall have a Shabbat of complete rest, a Shabbat for God: you shall not sow your field or prune your vineyard. You shall not reap the after growth of your harvest or gather the grapes of your untrimmed vines; it shall be a year of complete rest for the land. But you may eat whatever the land during its Shabbat will produce—you, your male and female slaves, the hired and bound laborers who live with you.

If the laws of Shmita are more detailed in Parshat Behar, then why did they need to be introduced in Parshat Mishpatim?

Shmita in Parsha Mishpatim is juxtaposed with the laws of Shabbat since both Shmita and Shabbat represent man’s testimony that God created the universe in six days and rested on the seventh.

Shadal writes in his commentary on Vayikra 25:2:

Just as the Shabbat day strengthens our conviction that the Jewish Nation is holy to God, so too, the Shmita year instills within us the belief that Eretz Yisrael is holier than all other lands. The land rests on the seventh year, just as God rested on the seventh day of creation.

And just as in the desert, God gave B’nai Yisrael a double portion of manna on Friday to last for two days, so too, when they are on their own land, they will receive the blessing of the sixth year that the land will produce enough for the seventh year as well.

Sefer HaChinuch, The Book of Mitzvah Education 84:2 explains:

At the root of this commandment lies the purpose to establish in our heart and set in our thought a firm conception of the doctrine that the world was brought into being as a new entity, out of nothing- As it says in Shmot 20:11: “in six days did God make the heavens and the earth” and on the seventh day God did not create anything - God imposed rest on Himself.

In order to remove, uproot and eradicate from our thoughts any concept of the world’s timeless pre-existence in which the deniers of the Torah believe in, through which they destroy all its principles and break through its walls - did the requirement come upon us to expend all our time, day by day and year by year, for this matter, by counting six years and resting on the seventh. In this way, the matter will never depart from between our eyes for all time. And this is similar to the manner in which we count the days of the week by dividing them into six days of work and the seventh is a day of rest. 

Therefore, God commanded us to render ownerless all that the land produces in this year - in addition to resting during the year- so that a person will remember that the land which produces fruits for him every year does not produce them by its own might and virtue. For there is a Lord and Master over it and over its owner - and when He wishes, He commands the owner of the land to render the fruit ownerless.

There is another benefit in this matter - to acquire the trait of letting go of one's possessions, for there is no one more generous than he who gives without hope for recompense.

And there is another benefit - the outcome of this is that a person will add to his trust in God, since anyone who finds it in his heart to give and abandon to the world all of the produce of his lands and his ancestral inheritance for an entire year - and educates himself and his family through this for all of his days - will never have the trait of stinginess overcome him too much, nor will he have a deficient amount of trust.

Celebrating Shabbat each week and observing the Shmita year once in seven years are constant reminders that God created the world and that He is in charge of everything. Just as the Jewish nation is holy, so too is the land of Israel and it must not be taken for granted.