This book of the law shall not depart out of your mouth, but you shall meditate thereon day and night, that you may observe to do according to all that is written therein: for then you shall make your way prosperous, and then you shall have good success.
Josiah's reforms led to a popular observance of the sabbath, and of the Passover, (2 Kings 23:22).  How did they know how to keep such laws?  Where else but in the Pentateuch could the instructions be found?