Mary, Queen of Scots ‘locked’ final letter using paper-folding, research finds (theguardian.com)

Locking letters are still useful for slowly-defined long-distance (or long delayed) coordination of planning a future covert activity.

If the lock is broken, then assessment is made to determine alteration of such planning (over long distance).

Socially, it would be useful in leaking rumors or revealing blackmail for others to know that such info were given to them in confidence.