Decompressing your WMA files to WAV will take a tremendous amount of space. The advantage is a very small increase in speed when the Play button is pressed. The Install button will convert all WMA files in your Drums folder to WAV. You can selectively choose a drum kit with the '...' button next to Install.

I happen to have a 9 year old XP machine. I have selectively decompressed the few RealDrums that I use. This saves a few seconds of time when Play is pressed. Newer machines probably won't see any benefit at all. You can experiment by comparing generation time with WMA and WAV.

Again, to Install the entire Drums folder is overkill. I would not bother unless you have a very slow machine.