Made from flour, milk and eggs it was originally used as a first course filler for poor people who could not afford much meat but is now almost always served with a roast beef main course to form part of a 'traditional English dinner'.
In its home county it is often served on its own filled with onion gravy so that it acts as a form of edible soup bowl. This practice probably originated with people who could not afford crockery either.
It has become very popular with tourists in search of traditional English fayre and ranks along with fish and chips as an internationally known item of English cuisine. In fact many foreigners believe the English national dish is 'Roast Beef with Yorkshire Pudding' but as we all know it is really Curry.
Generally speaking Yorkshire Pudding is best avoided by true food lovers.