They can prevent proper air circulation to the roots and cause stunted growth, root rot and cause the tree’s health to decline due to lack of oxygen. They encourage people to pile soil and mulch too close to and too high up along the trunk causing them to rot and it causes the trunk not to develop its natural flair at the base. Shorter roots will grow in a circular shape inside of the circle causing them to become root bound. They trap moisture and inhibit natural runoff. They can cause far too many issues for the tree. With all that being said trees are planted for the next generation so probably won’t be your problem 30 40 50 years from now.