Some of them are there, I think, so that if you take a letter to a branch (remember those) or it is returned, there is a quick way of indentifying the source an

Some of them are there, I think, so that if you take a letter to a branch (remember those) or it is returned, there is a quick way of indentifying the source and authenticity and content of the letter without lots of effort. Ie why you were sent an offer for a stuffed goat from the utility company... (UK stamps have codes on too, to reduce fraud I assume.)