Why navies use opaque names such as Force H and Force Z

Why do naval forces receive letter names such as Force H or Force Z?

The letters are deliberately opaque, avoiding the geographic and mission clues that a descriptive name would reveal.

A label such as Force H does not disclose where the formation is based, what it contains, or what mission it serves. A descriptive name such as a Gibraltar response squadron would give an observer useful intelligence before the ships even sailed.