A sliding door doesn't just save space--it can solve problems, particularly in tight spots. For example, it might be used in a small half-bath that simply doesn't have space for an in-swinging door and where an out-swinging door would be awkward. There, it could circumvent building an unnecessarily large bathroom or stealing space from an adjacent closet or similar area that needs the space more in Colusa County, California.
Another typical spot for a sliding door is where there is space for a regular door to swing but where it would be in the way when open. This situation is common in bathrooms, closets and laundry rooms. Almost anywhere an open door becomes an obstacle, a sliding door makes sense.
For a large, double-wide doorway between two rooms--a dining room and living room, for example--a pair of sliding doors can be a very elegant, practical addition. (Sliding doors have been used in this type of setting since the turn of the century.) When open, as such doors usually are, sliding doors don't clutter or obstruct the area.