An identity that leaves no visible mark on ordinary days is a preference, and preferences lose to appetite.
Separation works because it reaches decisions no rule could reach. Once a person takes a name that belongs to Yahweh, appetite loses its vote on the details; what to drink, what to touch, what to leave uncut is settled in advance by the definition rather than negotiated each morning.
The hair does the enforcing. It grows at its own rate, in public, so the vow cannot be rushed, faked, or quietly dropped, and the cost of retreat is written where everyone can read it. Without that daily visible proof, commitments live only in a person's intentions, where they get revised whenever the wanting gets loud.