I use paper (for my lists and other portions of my system) for the following reasons:
1. I get tired of using a computer all day. My biggest issue is that I can only see a small portion of information on the screen unless I zoom out and then everything is too small.
2. Paper is flexible and more interesting. I can do nearly anything. If I get bored I just change the format or the paper system I am using, e.g. planner, notebook, disc-bound, small, big, etc. If you find you need to do a lot of filtering and parsing of information, your system is probably overcomplicated. I am at the mercy of what an app can do, or worse I feel I need to use all the features of an app. Take a look at the GTD study guides. They only recommend using portions of an apps capabilities because anything else overcomplicates your system.
3. Using paper is usually satisfying. When I was on the digital roller coaster, it was exactly that. See the newest tool that is different (e.g Notion), and I gotta try that. With paper I just switch the "system" without changing the "system".
4. There are a lot of digital productivity "gurus" that are using paper - Shawn Blanc, Joe Buhlig, Mike Schmitz, and even MPU David Sparks (to some extent) use paper in their system. In Buhlig's case, all paper except calendar.
5. My favorite reason for using paper is that paper slows you down, makes it easy to focus and be intentional., and be selective about the things on your list. I listened to a David Allen audio from a while back where he stated that much of what he captures is never organized because it has been trashed during CLARIFYING. A great reason to decide a project/next action is not worth carrying anymore is the fact that you have to re-write it.
6. I never lost parts of a paper system. And if I did I probably could recreate it over the course of a couple of weeks.
I could probably go on but I have rambled enough. These are obviously personal reasons. There is no war between paper and digital. Everyone has a preference and that's fine. What I will say is that when you are having GTD problems or you are not sure GTD is for you (in whole or in part), try an analog system first and see if you don't feel better - before jumping to the next app or abandoning GTD altogether.