The Book of Numbers is quite a mixture of forms of writing. It contains both legal texts as well as narratives. Today's reading (Numbers 15-18) is a good illustration of this diversity. Numbers 15:1-31 describes the sacrifice for unintentional sins; Numbers 15:32-36 narrates the story of the man gathering wood on the Sabbath; and Numbers 15:37-41 details the law about tassels on garments.
I don't know that you can always make a clear link between the legal sections and the narrative sections, but I think you can do so here in Numbers 15-16. The law for unintentional sins precluded an offering for presumptuous sin. That is followed by a story about intentional sin - the gathering of wood on the Sabbath, and the severe punishment that followed. Next comes the law about tassels, which of course were to remind the people to remember God's commandments and not follow their own vision of what is right - which sets the stage for the story of the rebellion of Korah and his cronies.
This pattern afforded Moses the opportunity not only to state the Law, but also to show the application of the Law. Further, it shows the extent to which God went to help the people to remember His Law. Objects like the tassels, or Aaron's rod that budded (Num. 17) were visual aids to remind the people to be obedient to His Law. Such clear commandments combined with visual reminders left the people with no excuse.