2 Nephi 9:25

wherefore he hath given a law; and where there is no law given, there is no punishment; and where there is no punishment, there is no condemnation; and where there is no condemnation, the mercies of the Holy One of Israel hath claim upon them, because of the atonement: for they are delivered by the power of him:

So he has given a law. Where there is no law, there is no punishment. Where there is no punishment, there is no guilt. Where there is no guilt, the Holy One of Israel can show kindness because of the atonement. They are saved by his power.

That’s why he gave a law. Where there’s no law, there’s no punishment. Where there’s no punishment, there’s no condemnation. Where there’s no condemnation, the Holy One’s mercy covers them through the sacrifice—they’re delivered by his power.