couching is great against heavily armored targets. These are usually slow and have a harder time dodging you.
It's also great against the heavy horses that take a lot of stabbing to kill without a couch. Keep in mind couch damage is dependent on your speed relative to your target. Even on a champ courser at full speed, a plated charger riding away from you will take almost no damage from a couch. Also heavily armored inf running away from you can easily survive a couch lance in the back.
I usually reserve my couch for unawares or shielders, since many shields are one shot by a couch. Like i said, its also useful for taking out heavy cav, but they will usually dodge if they see you coming.