The hip rotators not only rotate the thigh on the pelvis but more functionally rotate the pelvis on the weight bearing fixed thigh. Activities such as swing a golf club, and even just walking require some rotation of the pelvis on the weight bearing leg. While we don't need that much range of motion to walk, activities such as running, dancing, tennis, and many other sports can require more hip rotation.
Now the catch with hip flexion is that most of us sit in chairs and end up in a position of passive hip flexion and knee flexion (bent knees) and retain that position for many hours a day. We know now that our bodies process the movement or lack thereof and adapt to the shape that we most frequently inhabit, for better or worse. If you primarily flex the hips and knees and never fully extend them, you may have chronically short or weak hamstrings, limited range of active hip flexion and limited range of active hip extension, for starters!
The good news is that there are plenty of good hip stretches out there that you can do to relieve discomfort, decrease tightness, and increase mobility in your hips. Since your hips are involved in so many of the movements you make (both inside and outside of the gym) stretching them is a great way to keep them feeling good and ready to work for you. Add some of the 12 hip stretches Atkins demos below to the end of your workout, or spend 10 minutes each day just doing a few of them, to improve mobility in your hips.