Because it implies that the one the one that put them in that prison to fight for 2 years for their lives may have used them as literal experimental subjects for the medicubethingie that is poised to help so many and potentially save lives.
I thought it made Kayaba so much more human then the original series really showed.
The guy who caused the deaths of thousands and also took away 2 years+ from Kirito and everyone else was also the one who was able to give Yuuki and the others this opportunity, even if he wasn't directly involved in this arc.
Well it seems like everyone understands why this arc is most of our favorites from the books. I'm really curious if we'll get a season 3 or not (I have no idea how the ratings for this or the first season were) and when we'll get said season if they do end up doing it as the current arc (which is what 4+ books now?) is still ongoing as far as I know. I started reading them but stopped early on, really should get back into that at some point.