When parts are left exposed to the elements during storage or when in use, they can get oxidized due to moisture or rain. While in some cases (see: green oxidized coating on the Statue of Liberty), that oxidation can provide a protective coating while in other situations (see above), the oxidation needs to be removed to clean and re-use the part.
The pictured part here is an extreme example of a rusty, steel part. Abrasive blasting can certainly be used to remove the rust but that is often a manual, one part a time process. Vibratory tumbling a batch of will remove the rust, flush it away from the part, and leave a clean, white-metal surface finish.
This part was tumbled with Ceramic Media and the Kramco 1510 Rust Inhibitor Liquid Compound for only a couple of hours. Rinsing the part in a solution of the Kramco 1510 after cleaning will help to prevent rust formation upon storage.