Couches receive a lot of friction from people sitting down, getting up and shuffling around on the sofa while watching TV and partaking in other activities. Objects in people's pockets such as car keys make the problem worse because they have sharper edges that will rub against the couch upholstery repeatedly. If the couch has thin fabric, it will quickly show signs of wear and can even tear outright, leaving an ugly gash in the couch that requires repair and possibly a full reupholstering.
Some couches have legs screwed into the bottom of it. If the legs pieces are poorly constructed or if the base of the sofa can't hold on to the screws effectively, the legs can warp and possibly break off. The more weight placed on the couch on a regular basis, the more likely the couch's legs could start having problems. Once a leg breaks, you should replace the couch or take all the legs off so the couch simply sits with the base on the ground.
Couches often have food near or on them since many people like eating and drinking on the couch. As a result, stains can easily appear on the couch. Some fabrics are more stain-resistant than others and darker sofas make it harder to spot stains than light ones. Not only are food and drink stains a problem, but grease from your hair and sweat from your skin can also discolor a couch over time if you spend a lot of time laying on it.
A couch needs to be comfortable for it to be useful. Some couches have more padding than others and everyone has their own preference regarding what they need for maximum comfort. A couch that is unpleasant to sit on will often be cheaper, but it can lead to back problems the more you sit on it. If you use your couch as a guest bed, a painful crossbar can ruin your guest's sleeping experience and the couch may need replacement if you can afford it.