Depending on what that couch has been through, it might be about marking/refreshing territory.

But also understand that there are different kinds of scratching materials and structure. My cat only likes the cardboard kind in a small A3-ish box and that is more because she likes to move it around. For actual scratching she very much prefers the ropier material in something sturdy at a 90 or 45 degree angle that she can really push against without it moving.

And I've NEVER seen a cat actually like one of those free standing scratching posts after the first time they push it over during a more energetic session.