As Michael says the presence of a external ferrite around a cable is an indication that a unit has shown a marginal fail / pass when initially tested. Often if the problem is being radiated by the cable(s) then ferrites can help, as i recently saw on a product that is designed for us by an external company. However we do not like to use them long term and often will only use them for initial shipments until a "proper" fix which is usually a cost saving measure as well is applied to production units.

Like wise they also prevent noise getting into a unit being tested, one test equipment manufacturer actually supplied ferrites with the equipment for this purpose, and while testing a ethernet based product we had a problem with noise which could not be found on the unit under test, and was identified as coming into the test chamber on the test equipment cables, a few ferrites applied to the test equipment end saw an end to the problem, and a very good compliance obtained.