The term 'couch' originally came from the Old French noun 'couche', which means "to lie down". Because of this, the term 'couch' was primarily used to describe a piece of furniture designed for reclining, lying, or sleeping on.
For example, the couch you recline in a psychiatrist's office will generally have a reclined back that you can lean against, and either just one arm or no arms at all, like a chaise lounge.
The confusion arises because the word couch is often used in Australia, North America, and South Africa to describe a sofa.
People will often use the word couch when describing comfortable, informal seating in a casual setting, somewhere you might watch movies and binge on popcorn and wine. The word 'couch' itself also sounds more informal, reminding us of the word 'slouch.' Somewhere you can truly relax. Because you can recline on a couch. Or even sleep if you want to!
The word sofa, on the other hand, might be used to refer to a more structured designer piece that you would sit on to spend time with friends and family, socialize, and relax.
This example shows what a couch would have been thought of in a historical sense. A comfortable piece of upholstered seating furniture that can be used to lay down or recline on.