Doing sets with the other swimming strokes has lots of advantages. Check out some of them thanks to Open Water Source
Incorporating other strokes in your workouts has the following benefits:
Butterfly helps build strength, is good for your abdominal muscles and is aerobically challenging.

Andre Saunders is developing a pretty good butterfly stroke
Butterfly and breaststroke require bilateral symmetry – because your left side does the same as your right. As a result, these strokes help establish more muscular symmetry.
Because backstroke is essentially the inversion of freestyle, working your arm and leg muscles in the opposite directions is helpful to building overall strength. Additionally, your hip rotation in the backstroke can help you improve your hip rotation in freestyle.
If you have a relatively weak freestyle kick, working on these other strokes, especially backstroke, will help you build your leg strength. Most swimmers kick more in backstroke than they do on freestyle.
Learning and working on swim strokes other than freestyle helps you become more kinetically aware of your body’s movements in the water which can help improve your freestyle.
Individual medley (butterfly + backstroke + breaststroke + freestyle) sets are aerobically challenging.
You can add significant variety to your pool workouts and become less dependent on hand paddles, pull buoys and kickboards.
Visit Open Water Source for more information about the world of Open Water Swimming