If sFTP is used, then is SSL needed?

If I am successfully connecting and transferring data using public/private key pair over sFTP, is there still a need to use SSL security to establish the connection?

In other words what, if any, additional value is SSL adding to an sFTP communication sequence? And if there is value with SSL, does it stand to reason that it would be even better if I leverage SSL for ALL communication (control and data) versus my current setting of just CONTROL ONLY?

As far as I know ssh is always encrypted.