The primary advantages are that there is no cost and the costume can be modified if required. There really are no disadvantages except that being limited to stock costumes may require some artistic compromises. Most professional costumers consider borrowing both unprofessional and unnecessary. The advantage is obviously that there is no cost, but the risk of damage and replacement expense can undo that. Actors will sometimes request that they be allowed to wear their own clothes.
Costumers hate it when stars insist on dressing themselves.
During the research phase of the design process, costumers undertake the same sort of collecting of visual materials that set designers do. Since clothing is the only subject, their search is more narrow. Most costume designers have accumulated their own personal libraries of source books on historic costumes and patterns. These source pictures are usually brought to early meetings with the director to help determine the look that is required. In the case of a contemporary show, current magazines are the preferred source.