It is always important to know the source of a specific Mitzva. An example of this comes from the words in אחרי מות where it's written ״וחי בהם״, that we are to live by the Mitzvot and not die by the Mitzvot.
Two important rules come from these two words. The first is the concept of פיקוח נפש, that if someone is in danger, one may violate certain commandments such as Shabbat and Kashrut.
The Talmud in מסכת סנהדרין quotes Rav Yochanan who describes an important vote among the rabbis in the city of Lod. It was there that they came out with the ruling regarding Kiddush Hashem based on the words, "וחי בהם".
They ruled as follows: All of the sins of the Torah, if one threatens a Jew with death or violation of the Mitzva, he should violate the Mitzva, and live. However, if the same threat is made regarding murder, idol worship, or immorality, one should allow himself to be killed rather than violate. Kiddush Hashem, sanctifying G-d's name, outweighs the וחי בהם when otherwise, it would be a חילול ה׳, a desecration of Hashem's name.
A great deal of very significant Halacha is learned from those two words, וחי בהם.
Yesterday was 28 with ספירת העומר