I believe for the most part that seiza (or suwari waza) technique is taught to assist in learning to move through your hips, not through your legs, giving you a more balanced approach when applying a technique.
I find that often you can see the particular details of a technique better in suwari waza as you are forced to actually think through your movements, rather than relying on years of experience walking on your feet.
I was just wondering if Aikido should stay static and keep training in the old way or should Aikido adapt and change and incorporate training from sitting?
As an interesting exercise, try a technique from a seated position; I think you will be surprised at how similar to hanmi han tachi technique it feels. From what I understand of hanmi han tachi, it is designed to teach you the practicality of performing technique whilst seated or in a lowered position, or to defend against a person that is taller and/or heavier than you.
Whilst sitting and kneeling are really two quite different positions, I think that suwari waza training is a good preparation exercise for seated attacks, and also benefits your normal waza...