I dunno... Why does Apple invest money in developing smooth software on the iPad / iPhone? Or why did Google spend so much time in "project butter" to improve android's performance (60fps smooth triple buffering).
Why did Microsoft spend so much development in Aero? Then when they did Windows 8, they still spent a lot of time on the looks and transitioning in and out of screens (live tiles / apps).
It's all about the overall experience. Does a graphic transition make it faster? Maybe not... But it provides a sense of context and makes it "feel" faster by visually executing an action at the push of a button.
I guess I just take modern day systems for granted.
Windows 8 is horrible but the performance is there