It is important for some people to bend their knees during Uttanasana if their lower back is higher then their sacrum. If this is happening they will probably be feeling the stretch primarily in their low back and not in their hamstrings where this stretch is designed to open.
By bending their knees they are able to gain more mobility in their pelvis, as the hamstrings are able to relax, and the pelvis can tip forward. They also ensure that the muscles of the lower back, such as the Quadratus Lumborum and the deeper Spinal muscles do not get over stretched.