Somehow, the idea that it's admirable to have "buns of steel" has become entrenched in our culture. It's true that these muscles need to be strong, but if there's too much steel back there—in the form of short, tight muscles—you're likely to face a multitude of difficulties.
In everyday life, those can include sciatic nerve problems, low-back pain, and even knee trouble. That's because tight buttock muscles limit the range of motion of the hips, leaving the low back and knees to compensate. Tightness in the posterior hips can also pull the knee out of alignment, thereby contributing to knee tendonitis and even arthritis. In yoga, you're likely to come up against frustrating flexibility challenges, particularly in forward bends and seated twists.
Most often, the buttock muscles get overly tight when you spend a lot of time sitting, especially with crossed legs or ankles. Activities like cycling and running can also contribute. Whatever the cause, the solution is to do stretches and yoga poses that lengthen these muscles and teach them to relax. If you can squeeze in some butt stretches a few times a week, not only will your low back and knees be happier, but you'll be rewarded with greater ease in your practice.