This seems to be pretty easy but after a long Google search, I couldn't find the actual reason (or benefits) on adding an extra clutch plate.

Could someone explain the pros and cons of doing so?

 

The extra friction plate will help the clutch hold better. Not a bad idea for a moderate engine build, along with a stiffer clutch spring. Downside would be cost if it is not necessary in the first place. The stiffer clutch spring that usually goes along with a +1 kit will cause a harder clutch lever pull.

 

It used to be fairly common to add and extra drive and driven plate many years ago. It resulted in less clutch slippage and heat when launching. New friction materials have sort of rendered it a very minimal gain anymore.