So what do folks focus on in suwari practice? What does it lend itself to? Why do we have suwari practice in Aikido?
The focus for me is purely and only the learning of energy harmonization. The receiving of the 'force', (to center) (accepting) and then the redirecting of it.
For instance, you get a person pinning your wrists to your thighs, you learn to do tenchinage from that position. Now technically, you would have to bow forward slightly, let the opponents force say push your left wrist down (by letting it slide off of the outside of your thigh) meanwhile with your right hand you turn the force in and up center line and out to a circle. Turn them, moving from the knees, maintaining the flow, and pin.
Energy manipulation. Learning the ways and paths of energy motion. Learning the ways and paths of harmonious energy motion. Learning the ways of using center. (and koshi for that matter)
These exercises can be made more and more difficult where the partner is allowed more and more to try and block whatever you are trying to do, however you are trying to harmonize and lead and thus can be done at all levels.
You learn to eventually feel where the 'openings' are and they are always to do with the rules of energy motion.
So that's my use for suwari waza. It's also therefor a great Ki development exercise.