Staff Letter Boxes on Platforms

What were they actually for? Quite literally sending staff letters from the general public or was there a 'Tube Postman' who would hop on and off trains delivering the post?

I thought those built into the walls at Charing Cross were actually rubbish bins?

No not for letters from the public, they were for staff letters and notices, things that concerned the station they were delivered to, and yes we did indeed have staff postmen, usually jobs for the staff who had long term sickness issues.