The same reason judicial decisions (especially appellate ones) applying those laws are also put in writing, and the same reason that such decisions are binding on lower courts, and considered persuasive though not binding authority for other courts in applying the same law.
For reference and consistency.
The same reason judicial decisions (especially appellate ones) applying those laws are also put in writing, and the same reason that such decisions are binding on lower courts, and considered persuasive though not binding authority for other courts in applying the same law.