"We've never thought "Why the useless letters?" as, if I recall correctly we (in the UK) have pretty much always had them on phones (fixed or mobile)... they've always just been a 'part' of a phone."
Well speak for yourself, because in the years before mobiles and text messaging I remember looking at the buttons and wondering why there were letters on them as well as numbers. I even tried dialling "names" when I was much younger, usually to either get an unobtainable tone or a slightly miffed person on the other end. And I'm talking about dialing on one of the big old phones with the circular dials.
Of course, if you're in your late teens/early twenties, you will probably barely remember the days before texts.