Rather that thinking about "Why?", it is much more helpful to look at "How?"--which really means examining how the high string it is used while playing. And, though it may seem to make sense to someone who is trying to grasp this in the abstract, Artful Codger's technique isn't really a very important one in the world of uke playing.

Check this--Jake Shimabukuro shows how to pluck strum

And this--A great strum pattern