They are used to control the spread of oil to reduce the possibility of polluting shorelines and other resources. They also concentrate oil in thicker surface layers, making recovery easier.
They are also a strong mesh sleeve encases a poly sock skin filled with highly-sorbent polypropylene which repels water while only absorbing oil and other hydrocarbons. It doesn't sink even when saturated with oil. A nylon rope and sturdy hardware ensure long-lasting durability and easy retrieval too.