Pantone talks about why and how they came up with the Plus Series. It's all about color.
Designers were asking for more options in the Pantone Library. If you've ever tried to match a screen color in print, you know how difficult and most times impossible it is. This doesn't solve those problems, but it does give you more options to choose from when picking/matching colors and may enable you to get a little bit closer to the screen version of a color. Did you know colors are infinite? We can never have enough color!