On no account shall a monastic be allowed to receive letters, blessed tokens or any little gift whatsoever from parents or anyone else, or from her sisters, or to give the same, without the Abbess's permission. But if anything is sent her even by her parents, let her not presume to take it before it has been shown to the Abbess. And it shall be in the Abbess's power to decide to whom it shall be given, if she allows it to be received; and the sister to whom it was sent should not be grieved, lest occasion be given to the devil. Should anyone presume to act otherwise, let her undergo the discipline of the Rule.

What is Benedict's purpose? Fairness within the community or separating the monastic from the outer world or discouraging attachment or...?

He does not explain the principle behind this rule. I can perceive its utility on various counts. But I would be more inclined to accept the discipline if I knew its purpose. I might also argue against certain principles.

Without principle or purpose how does the leader discern when to allow and when to disallow?