This can be a very effective treatment for a slave who does not use her mouth wisely. Whether she is disrespectful or she simply speaks without thinking, a gag is a very effective reminder that helps keep slave in her place.
Gags are also useful for the loud slave.
If slave is given to emotional displays during sexual use or if slave is being punished and Master does not wish her screams to be overheard, then a gag is a perfectly appropriate solution. Care must always be taken so the bound and gagged slave can breathe properly. My own preference is to use perforated gags so that, even if slave's sinuses do clog, she can still breath through her mouth. Besides, I enjoy watching her drool. She, of course, is totally humiliated when that happens.