Best reason for me is quick follow up shots for predator hunting.Is your 20PPC a gas gun?
I have a 6mmAR.com built 20 Practical. Very very accurate for a gas gun. Just like the article on Accurate Shooter.
That said. I'm not fond of gas guns, I don't believe in "Black is cool". There has to be a reason beyond AR, I use them for PRS gas gun games.