The pack drag is a low capacity anchor used to slow a decent but not rappel from. It is used to convert an uncontrolled slide into a controlled slide.
The pack drag is fast and convenient.
The pack drag takes some guesswork and experience to use properly. If you wedge the pack too tight, you might not be able to pull it down once you're at the bottom. If you don't wedge the pack enough, it might come lose and you could slide out of control.