Self defence comes to mind: Historically, and I guess still now-a-days, people do sit in seiza in Japan. It is not an idea position to defend oneself from and this is by design. Therefore, daito-ryu (and many others) developed techniques that could be used to fight either a sat or standing opponent. Most of those techniques emphasis getting up as quickly as possible to either escape or use one's blade whichever was available.
I suppose that you might encounter a situation where you have to defend yourself while sitting in seiza. Although, I consider this unlikely in the extreme. Thus, the self defence aspect of suwari waza is more or less of historical interest.
Suwari waza is about isolating your hips from your legs. When kneeling down, you cannot "cheat" with your legs: all the power must come from your core. This, to my mind, is why suwari waza is still useful as an exercise. It teaches that economy of movements can have massive impact on the opponent provided that said movements are focused.
The first eight technique of the shodokan goshin no kata illustrate this. In the first four techniques, you see both the traditional four timing points and techniques that are performed very close to uke with minimal movement from tori. In the next four, you see selected attacked from each direction, leading to tori standing up at the end. Note that I have seen this eight done standing up by old people with shot knees. There is nothing magical about doing them kneeling down, but for distance and economy of movement.