ב דַּבֵּר אֶל-בְּנֵי יִשְׂרָאֵל וְאָמַרְתָּ אֲלֵהֶם כִּי תָבֹאוּ אֶל-הָאָרֶץ אֲשֶׁר אֲנִי נֹתֵן לָכֶם וְשָׁבְתָה הָאָרֶץ שַׁבָּת לַיהוָֹה:And it says over there (Shemos 20:10),
Speak to the children of Israel and say to them, "When you come to the land that I am giving to you, you shall let rest the land, a rest for G-D."
וְיוֹם הַשְּׁבִיעִי שַׁבָּת לַיהוָֹהThese similarities can not just be a coincidence, there is some connection here that will help us understand the purpose of Shemittah as it relates to Shabbos.
And the seventh day shall be a rest [day] for G-D.
The Ibn Ezra tells us on our Parsha something very vague (Vayikra 25:2)
וטעם שבת לה' - כיום השבת, וסוד ימי עולם רמוז במקום הזה.The Ibn Ezra helps us out here by telling us that Shemittah is related to the Shabbos day. No surprise there, but when we are looking for reasons for the Mitzvos this is a good starting point. We do not need to come up with some roundabout idea that references obscure ideas about the land and what not. The idea behind Shemittah is simple, it is another way for G-D to drive home a point that we are supposed to learn through observing the day of Shabbos. However, what is this point?
The reason it says Shabbos for G-D is because this is like the Shabbos day. This is a reference, in this place, to the secret of the days of creation.
The Ibn Ezra goes on, in his commentary on the Ten Commandments (Shemos 20:8), to say that Shemittah comes to teach the same idea as Shabbos. However, he then goes on to explain the reason behind Shabbos and Shemittah.
והנה השבת נתנה להבין מעשה השם ולהגות בתורתו. וככה כתוב: כי שמחתני ה' בפעליך.The point here, as clarified by the Yaheil Ohr (a super-commentary on the Ibn Ezra), is that the point of Shabbos and Shemmitah is so we can take time out of our busy lives and learn Torah. This is brought down in the Talmud Yerushalmi (Shabbos Chapter 15:3) in the name of Rabbi Berechia in the name of Rabbi Chiya bar ba. It says there,
Behold, Shabbos was given in order that we should understand the works of G-D and to study His Torah. Like it is written (Tehilim 92:5), For You, G-D, have gladdened me with Your Works.
The only reason Shabbos and Holidays were given was so we could deal with the words of Torah.Just as a side point, I personally think that this teaches a very valuable lesson. Shabbos and Shemittah show us that during the week and during the six years before Shemittah we are supposed to be dealing with things that pertain to sustaining our lives. It is only during Shabbos and Shemittah that we are not supposed to deal with anything except learning Torah.
I concede that this could also mean something entirely different. It could mean that there are those of us who can not find someone else who will support us and allow us to just learn all the time. If that is the case, we must support ourselves during the six years before Shemittah and during the week. However, even if no one else is supporting us, we must take the Shabbos day and the Shemittah year and spend that year learning the Torah.
I think either of the previous explanations is plausible, but I think the first makes more sense because if you do not work during the six years before Shemittah or during the six days of the week, how are you supposed to appreciate Shabbos or Shemittah? What is the point of Shabbos or Shemittah if you are always learning?
Another explanation for the reason for Shemittah can be found in the Yalkut Peirushim Al HaTorah (Conglomeration of commentaries on the Torah). It says as follows (in the name of the Tzarur Hamor Vayikra 25:2):
Just like we are obligated to honor G-D and to honor other people, so too we are obligated to honor the land of Israel.This idea can be understood much better and can be connected to Shabbos if we take the explanation of Shadal (also found here in Vayikra 25:2)
The commandment of Shemittah is similar to Shabbos because, just like the commandment of Shabbos is to reinforce in us that we are a holy (chosen) nation, so too the commandment of Shemiitah is supposed to reinforce the idea that the land (of Israel) is a holy (chosen) land... The holiness of the land is a reason for the nation to distance themselves from impurity and desecration with disgusting acts.This second idea seems to be telling us that the holiness of the land and the holiness of Shabbos teach us two similar ideas. Shabbos teaches us the Jewish people should rest on the seventh day because they are a holy nation, therefore, they should not act immorally. Shemittah teaches us that the land of Israel is a holy land, therefore, we should not act immorally. Both ideas come to teach us the holiness of the Jewish people, however, they teach this from different angles. Shabbos teaches the Jewish people's holiness with regards to the individual. However, Shemittah teaches the holiness of the collective nation, because you can not act as an independent nation without a land.
These two ideas, that Shabbos and Shemittah were given so that we should learn Torah vs. Shabbos and Shemittah were given to teach us that we are a holy nation appear to complement each other. The first idea, learned from the Ibn Ezra, tells us what we are supposed to do on the day of Shabbos or during the year of Shemittah. The second idea, that of the Shadal and Tzarur Hamor, tells us what the day and year themselves actually teach us about the Jewish nation. The fact that we are supposed to learn the Torah on these days is because we are a nation of individuals that learn and follow the Torah and THAT makes us a holy nation.