It's only really a thing in long haul, where these days there are usually at least 4 classes: economy, premium economy (same cabin, slightly more legroom, sometimes a larger entertainment screen), business and first (and sometimes tiers of first e.g. cabin class).
Premium economy IME is not just a little more legroom, its more akin to a domestic first class experience. Wider chairs, better meal, better booze selection, etc...
Note that "Premium Economy" is a newer term being thrown around and is different than Economy Plus, Comfort Plus and such that airlines throw around to denote the same economy seats with an extra 6 inches in front of you and maybe a free drink/snack.
