Yes, I'm well aware of that, Plasma can be installed on many distributions. How exactly is K.D.E. making their own software distribution benefiting the development of Plasma?

This really feels like they first decided they wanted to make their own distribution for no reason, and then went to search for the niche for it to fill which is the wrong way to do it. Usually someone, who typically does not do anything else at the moment, realizes there is a certain niche that needs to be filled and then decides to make a distribution that fills that niche.