I’ve seen more than a few shooters who’s phone has went down during a match. Broke, battery died, extreme cold weather. Then they have to deal with the pain in the ass of asking a friend to inout all their info into friends ballistic app to try and stay on track. Ma’am’s thing for a hunt. Dope books are free, and they’re easy. Don’t need to make it over complicated but they’re great for going over previous scenarios and situations as well has having hard data written down.
Yeah, phone batteries die, but books get lost, torn, wet, etc too. Personally I'll stick to having dope cards attached to my rifle instead of needing to consult a separate book. Lots of ways to do that, and if the card is missing, the rifle probably is too so the dope doesn't matter.
I've seen some guys keep a set of 4-5 laminated dope cards hanging on the rifle for 2K ft (or whatever) DA changes, but personally I prefer to have just one chart taped/glued directly to the stock or scope. In a couple cases I've just written directly on the gun with sharpie.
To each their own of course and some guys love digging through a book and writing notes after each shot (or were just taught to do that). I don't, and prefer to just record things like max distance to lands, suppressor POI change, cold bore impact, etc but don't track every shot and the book doesn't usually come out until after the shooting's done. Not saying I'm anyone's expert either though, that's just how I do it.