Sight is a powerful sense that's necessary when it comes to working out, but sometimes it takes over and can actually make your workouts easier. Take away your ability to see, and you have to rely on your other senses to move your body. I wouldn't recommend running or biking with your eyes shut since you can end up seriously hurting yourself, but it's great to try when strength training. Balancing becomes more challenging, and you can really concentrate on your muscles and how to move efficiently. Closing your eyes can also allow you to focus more on your breath, which is often overlooked when doing strength exercises. I also find that shutting my eyes calms my mind, so it's also a great way to relieve stress. This isn't something you need to do every time you lift, but doing it every so often can be a fun way to mix things up.