Well they mainly work out your pectoral and triceps, some benefit also goes to the shoulders, delts, and midsection.
Press Ups are a good BW exercise, just doing them really won't build muscle, specially if you already know how to do them well. If you keep doing them you will, however, build endurance.
I would say if you actually want to use them as an exercise add some weight to your back and do them, that's what some people do.
Also try different variations of press-ups, e.g. wide grip, close grip, diamonds, declined, etc...

You'll never outgrow the pushup. In terms of muscle building and pure strength, resistance training is superior, but pushups always have a great place for people of all ages and abilities. They make a good warm-up to get the blood pumping and muscle/joints ready, as well as getting the heart rate up. They're portable, scalable, and always challenging. If standard pushups, elevated feet pushups and diamond pushups are too easy, try doing some on a frictionless plane (using TRX or olympic rings), try doing some handstand pushups against a wall, or freestanding, or learn how to do a planche pushup. No one ever outgrows the pushup.