Take some examples of laws that have changed over time. Say, interracial marriage. It was illegal in many places to marry someone of a different race. If this had been perfectly enforced, no one would have ever dated or see couples of different races, and people would have had a lot harder of a time exploring and realizing that the law was wrong.
The same thing could be said about marijuana legalization. If enforcement was perfect, no one would have ever tried marijuana, and there would have never been a movement to legalize by people who used it and decided it was not something that should be banned.
We need to be able to push boundaries so they can move when needed.