Cells polyubiquitinate polypeptides to signal their destruction, either because the proteins are badly folded and, thus, non-functional, or because the proteins are no longer needed.
Work Step by Step
Ubiquitin is like flag you might put on items in your house when moving. Some items would be flagged to be thrown away (a broken DVD player--why bring it along on the move?) or others because you no longer want them (an ugly couch--put it in the moving sale).