Built in desks and bookshelves eliminate the need for annoying furniture moves. Many movers probably wish this was more of a trend in housing. It would save the hassle of measuring door frames and furniture to see if the pieces will fit through the entrance.
When a place has built in desks and bookshelves, it does not mean that additional bookshelves cannot be added to the room. Unless the room is a two person office, then people probably would not want two desks in the room. However, there is always use for an extra desk in a room that might not have initially had one. Our lives are becoming more and more digitized, so most everyone has a computer for their own personal use as well as a laptop for transporting.
These features in houses and apartments are not common, but sought out by many people who are moving from across State and find it hard to transport all their furniture to a new location.