Couches are more versatile than chairs. Some things are just more comfortable with a couch. A chair, on the other hand, offers some limitations. Actually, neither really does very much. But, the couch looks more impressive not doing anything.
Couches offer much more room for sitting.
Trying to put an entire board on a chair is difficult whereas stacking them on a couch is more easily accomplished. Chairs are not as predictable as couches. You never know about chairs. Some go to the beach. And, some are for potty.