There are several different reasons why you might want or need a passport. Here are some of them:
You are going abroad
You are passing through Arizona and need proof of citizenship
You're going to Disney World and need a valid ID for your child
You are applying for a job and need proof of lawful residence and the right to work
You have no permanent place of residence and need proof of identity
If you would like to have your passport processing expedited, here is one option: