These breeds have a distinctive look: a long, slender body, long legs, a long narrow tail, a wedge-shaped face, fairly large ears and (often) a fairly long neck in short, traits you associate with the Siamese. They are shorthaired but distinctive enough to be considered as a separate group.
Think of the stout-bodied, round-headed, round-eyed, snub-nosed Persians as one cat extreme and the lanky, wedge-headed, slant eyed Siamese as the other. In the middle are the moderate-bodied, moderate-headed and moderate eyed shorthairs. However, as you’ll see in the descriptions of the longhairs, many of those breeds have Siamese/Oriental ancestry.