Sefer Vayikra begins with the Mishkan service, whose details will comprise most of the book. Though its devotion to this theme clearly distinguishes Sefer Vayikra from the other books of the Torah, it also links it to Sefer Shemot, which culminated in the Mishkan's construction and establishment as the place of God's residence amongst the Jewish people. In fact, on a deeper level, the Ramban points out that Sefer Vayikra really protects the accomplishments made in Sefer Shemot, in which the Jews were redeemed from Egyptian slavery and were promised that God would dwell amongst them. By providing the rules of the Mishkan service, Sefer Vayikra sought to ensure that God's presence would not be driven away by the sins of the Jews, which could now be atoned for by offering sacrifices.
But Sefer Vayikra is much more than a detailed description of the laws of sacrifices and taharot. The text will take us far afield, teaching us the laws of kashrut, the prohibitions on cheilev and blood, and the laws of incest and adultery. We will also learn many of the laws of tithing in Sefer Vayikra, as well as the laws of sh'mitah and charity. The Ramban explains that Sefer Vayikra included these laws because of its kind of "stream of consciousness" style, which leads to the study of issues that are tangential to the main themes of the book.
Nevertheless, the chosen style is troubling. Surely there is a more efficient format in which to communicate laws; the Rambam's Mishneh Torah, the Shulchan Aruch, and a host of other texts that have shaped our mesorah throughout history have all been carefully structured and organized. Why was Sefer Vayikra not written in the same way?
Rav Nisan Alpert zt"l suggested that by using this style the Torah teaches us several insights. In his sefer, Limudei Nisan, he speculates that by making us learn scores of halachot from the book of the chumash that is otherwise dedicated to the rules and regulations of the Mishkan service, the Torah underscores the connection between the Mishkan and talmud Torah. We are being reminded that just as the Mishkan service assures our connection with God, so too does the mitzvah of talmud Torah.
Additionally, Rav Alpert suggests that Sefer Vayikra--called "Torat Kohanim" by Chazal--includes many different halachot among the laws of the Mishkan to stress that the role of the kohanim is also to teach Torah. In the words of the prophet (Malachi 2:7), "Ki siftei kohanim yishmeru da'at" ("The lips of the kohanim will protect our wisdom); for kohanim, the physical duties performed in the Mishkan are only one facet of a lifestyle dedicated to teaching the Jewish people and tending their spiritual needs and growth.
Finally and most importantly, Rav Alpert saw in the structure of Sefer Vayikra the model for the style that we now view as characteristic of the mishnah and, even more so, the gemara. The free style of discussion which moves easily from one topic to another and often follows tangential connections suggests that students of Torah should be familiar with the wide range of subjects it contains. Not only are rabbinic laws modeled after Torah-laws, but we can now appreciate that the very style of the Torah she-Ba'al Peh is rooted in the style of the Torah she-bi-K'tav. The entire Talmud was composed in the style of the book of Vayikra, the only book of Torah she-bi-K'tav dedicated almost entirely to halachah.
Moreover, through this style of halachic composition the Torah informs us that no parshah (section) of Torah should be studied or practiced in a vacuum. No halachah can be fully appreciated without seeing it as part of a complete regimen of practices and behaviors. Any single halacha can have its intended impact on our spirits only when observed in concert with all of Torah. The mitzvah of se'udat yom tov, for example, must be considered alongside the prohibition against bal tash'chit; the permissibility of shechitah comes together with the prohibition of tza'ar ba'alei chayim and the mitzvah of shilu'ach ha-kan. The same legal system that demands the destruction of Amaleik or an ir ha-nidachat also appreciates the plight of the stranger, orphan, or widow. We believe in shabbat and in Torah-study, but we also have a work ethic. Any attempt to view parts of Torah in a vacuum, however efficient they may be, can lead to a distorted understanding of Torah. A ben- or bat-Torah should be raised from his or her earliest years to know that shabbat, kashrut, lashon ha-ra and geneivat da'at are all threads of a single beautiful tapestry which can tear and unravel if pulled in different directions.
Rav Alpert saw the same notion in Parshat Mishpatim. Immediately following the story of matan Torah in Parshat Yitro, the chumash there confronts us with the laws pertaining to such varied subjects as altar-construction, slavery, property rights, torts, theft, and idolatry, to name but a few. Its goal is to introduce us right after Sinai to the warp and woof of Torah Law, a massive legal body that God devised to address and shape different kinds of people in all walks of life, to enhance everyone's spirituality and consciousness of the divine.