I took the card that came through my door to the polling station, confirmed my name and they gave me 3 pieces of paper for me to put my crosses in.
Yesterday I went to the post office to pick up a parcel.
I took the card that came through my door to the post office, confirmed my address, showed the man my passport when asked for ID and they gave me my parcel.
In both cases I could have done the same without the card, it's just more convenient if you have it.
Although, I would obviously be quite upset if someone ran off with my parcel, it seems it would be far worse if someone stole my vote. So, what's stopping me (other than my personal morals) going into a polling station somewhere in the country and using someone else's vote, if I knew which polling station they were registered with and that they hadn't already voted?
It seems strangely inconsistent that I need to show ID for more and more things nowadays (although I'm not convinced that's entirely a good thing), but not for voting.