Setting is important in story telling, especially when you want to tell what time period that story takes place in. The easiest way to do this is to simply state what year the story takes place in... however, being too exact may sometimes narrow down flexibility. So in order to pinpoint the time and keep it vague at the same time, writers like to give the century number, but replace the year and decade with X. For example: 20XX. You know this takes place after the millennium, but when after the millennium? 2097? 2030? December 21, 2012? We don't know, and that's the beauty of it.