A coupe may be, and given production realities, often is, based on a sedan, but it's less practical and more fun and frivolous. If sedans are sensible clothing, coupes are fashion items. Sedans are family vehicles, coupes are built for two, with, sometimes, a small rear seat for short-term use. Styling is of greater importance than practicality. Some coupes are more about image than substance, and in the politically-incorrect past earned the nickname "secretary's cars." The late and mostly unlamented four-cylinder Ford Probe
was a fine example. Others have the power to back up the image, for a price. See various hardtop Ferraris and Lamborghinis for details.
Coupe popularity varies in seemingly random cycles, at least in the lower and middle portions of the price range. These days, the Ford Mustang is the most popular of the lot, with both style and power for an honest price. The Honda Civic Si gets the honors in the smaller-displacement class, with the BMW 3-Series Coupe and M3 variant established leaders in the medium to medium-high price (and performance) range. And there is plenty of Italian exotica for the well-heeled.