A locked diary assumes the future reader and the present owner may want different things.

Locking a private record acknowledges that a later reader might want something you do not want to give. The present self writes freely and protects the writing from a future that includes people who would use it differently, including, sometimes, a future version of the writer. Honesty needs a boundary to exist inside.

Which is an argument for keeping something genuinely unread. A page nobody will see, written without an audience, is where you find out what you actually think rather than what you would say. If everything you write is for someone, you have nowhere to be uncertain in.