Beyond anything to do with incontinence, my understanding is that they are intimately involved with back pain and are one of the most important muscle groups to strengthen, as flexing them (or not) influences the position of most other parts of the body. (When people say "walk tall", what they really mean is "turn your pelvic floor muscles on".)
Maybe it's just assumed that you must use your pelvic floor / core to do any exercise with good form?