A clear nose ring in used when you may want or need to concel your nose ring or stud. Body art and tattoos have exploded in popularity and many consider them a socialy acceptable form of self expression. However not every feels that way and sometimes it can hurt your chances for landing a job.
A retainer can be used and does a great job of hiding a nose piercing and unless someone looks closely they are almost invisible.
Retainers are also sometimes used when there is a chances of having a nose ring caught or snagged during physical activity.