Why are exiled fairies sentenced to collect interesting tales from the mortal realm?

There are two reasons.

The fey folk love stories and music. They are expert at telling tales and singing haunting ballads. What they cannot do, however, is create these things. Fairies lure storyspinners and musicians into the fey realm from time to time, but the story-gathering of fairy malcreants is the primary source of new tales. For this reason, exile is regarded as a form of service to the community.

The second purpose of exile is rehabilitation. Fairies cannot compose music or write stories because the structure inherent in these art forms is foreign to their nature. Fairies who are exiled for Deadly Mischief are more chaotic than most of their kind. Transcribing stories and ballads imposes order upon chaos.  After a time, some exiled fairies reach a point where they can safely endure the magic of the fey realm.

Fairy sages debate what might happen if one of these exiles gained the ability to tell stories of her own. Most agree that this would change the fabric of Faerie in ways no one can fully anticipate.