Her Britannic Majesty's Secretary of State Requests and requires in the Name of Her Majesty all those whom it may concern to allow the bearer to pass freely without let or hindrance, and to afford the bearer such assistance and protection as may be necessary.It said, in other words, "Don't mess with me. I've got the British government behind me."
These days it is hard to resist the impression that governments have more in common with each other than they do with their own people. Hence the enthusiasm for extradition, whether within Europe or across the Atlantic.
A passport now fulfils the function that school uniform once did. It tells the observer which authority they should complain to if they don't like your behaviour.