A good and timely question. The most basic answer is because the Torah says it’s a mitzvah (Lev. 25:1-7)! In addition, a renowned rabbi of 13th century Spain offered three reasons for this mitzvah. These are found in his famous book called Sefer HaChinuch (lit. “The Book of Education”), which elucidates all 613 commandments.
1) Not working the land all year and giving up ownership of the fruits of this year serves as a powerful reminder that whatever the land produces at any time is not merely a result of human efforts, but is determined by what God decrees. If God decrees few or poor quality fruits that is what will be; a decree of plenty and top-quality will result in an abundant, delicious harvest.
2) Allowing others free access to one’s private field for an entire year every seven years helps instill in the Jewish People the character trait of generosity. The farmer is giving up his fruits without any expectation of any type of payment in return. Observing the mitzvah of Shemita helps us acquire a greater concern for the needs of others and helps build a more caring, sharing and moral society.
3) Not working one’s land in the seventh year and not cultivating it for his livelihood shows that the person has full faith and trust in God to provide for his needs. This will help increase his faith and trust that God will always provide for him and his family.
