I often struggle with this but the fact of the matter is that hardware is getting smaller and faster all the time so for the majority of people a separate computer and monitor is not necessary.  Historically we used the "big box" computers to house extra hard drives, CD/DVD units, video cards, etc.  Now we use cloud storage and/or network storage and download all our files.  Hard drives used to be 3x5x1 and now are the size of a credit card (or smaller).