<< If you have to ask, you probably don't need em.
Actually, they are round and compact instead of flat. Major benefit is cleaner air flow in your case. They also give a cleaner appearance (if you care) and are easier to route. I know of no down side to them except maybe price. >>
I do it better for the best airflow. I route under the motherboard zip tied to the side before going to the drives.
Best airflow-better looking- less expensive.
Rounded cables would disturb the airflow so they are not that great.
Anyway my thoughts on rounded cables.