What does the man from British gas and UK Imigration have in common?

This one really confuses me, but:

Why is it my passport is as useful to identify myself to my Bank Manager/anyone I have known for more than 5years in the finance industry as travelling between countries?

And why do they always ask for a secondary piece of identification "such as a gas bill"?

Is British Gas now the home of UK 'identity card' policy? Why should the gas man know me as well as the man from UK Immigration, and has he done as thorough a set of checks? Does this now mean I can travel to Toremolenas on my gas bill - or any other utility bill come to that? Should I chose my gas supplier on how many people get refused under ID procedures?

If someone can answer this, I would be most grateful. It's not I don't understand money laundering requirements - but why a gas bill?