Why is it pour Paris and not à Paris?

I would like to understand when we use "pour" and "vers" instead of "à". Is it just with the verb envoler, or are there others. In English, I think it is a bit like "headed for", "took off for", "left for", or in nautical terms, "set sail for", though in English for this specific example, it is more comfortable to say "flew to" or maybe at a stretch "was en route to".