Although this artist http://www.painterly.co.uk/
has proven it is entirely possible to not be trained traditionally to work digitally there are still many advantages to having a regular sketchbook.
It's simple, you don't have to worry about much else than an eraser and writing utensil with paper. You don't have to wait for your sketchbook to load an OS and start up. It's ready when you got the pencil/writing utensil (some people like doing ink studies).
It's a bit hard to take your tablet every where you go and have it ready on the spur of the moment idea.
More likely to learn from mistakes
As much as I love undo and redo, it's a killer. Instead of working a drawing through, you just go in this constant undo/redo cycle than really solve problems. Pencil and Paper or mediums like Ink are less forgiving. This actually, I truly believes works more in your favor than doesn't when learning.
Most people are going to be unimpressed with your first drawing. Some people like giving the backpats of "it's better than what I can do" but really that's bs. Look, with very rare exceptions we all sucked. Just because you put salt, pepper, lettuce, and tomato, on a shit sandwich doesn't really make it anymore tasty.
It's going to suck if you don't have the right ingredients and that's learning foundations. This also takes practice. You might get discouraged because it takes a while, but then look at the many things we do today that is considered mundane or second nature.
(Granted what I'm posting below has various degrees of success with people but you get the idea)
Tying our shoes.
Being housebroken/potty trained.
Reading and reading comprehension.
Pouring liquids into containers without spilling them all over the damn place.
Walking without falling all over oneself.
Riding a Bike
Driving a Car
So these things took time, in fact things like reading and other types of education took years. It's the same with drawing, and like some courses at school some things you caught on faster than others. Drawing again is no different, some things you'll catch on some things will be more frustrating.
It is up to you if and how much time you're willing to work into improving.