You can call it "local color" if you prefer.
Besides the benefits of world-building, it can -- if it's the right kind of clutter -- combat one tendency to Mary Sue.
You can make it clear that she's not unique and the universal center of attention.
There can be suggestion of other people's magic things as well as the marvel the character has; if your character has several, we can hear of others who also do so. There can be prophecies about other people, places, and times. We can hear of feats performed off-stage that are marvelous as the character's -- perhaps a little more, perhaps a little less, perhaps the same, according to how skilled you want the character to be, and how early it is in his career. And people can talk about things that don't feature in the story and therefore don't relate to the character.
And -- it helps with the world-building, too.
Two birds with one stone, always useful.