Why do writers use appositives and appositive phrases in their writing?

Appositives help with description. Appositives consist of nouns, or noun phrases, that help to define the term they are used with. For example, in the sentence "John, the barber, cut the Mayor's hair today," the appositive is "the barber." It helps the sentence make sense by defining John's role. If you did not know John was a barber, you would probably be very confused as to why he is cutting the Mayor's hair! It also helps you specify different terms that can be applied generally. For example, if you just say, "The tree  is beautiful," you are not really creating an adequate picture in your reader's mind. If you say, "The tree, a weeping willow, is beautiful." Here, your appositive, "a weeping willow," gives your reader more information and paints a clearer picture for them. Hope this helps! :)