I do most of my rough sketches on the iPad these days. It is just more convenient and I can work from anywhere.
I sometimes start the sketch on iPad, then continue in Photoshop, maybe switch back to iPad because I am on the road, or I just want to use that one brush on Procreate for a very specific texture.
With a single image, I can take it to finish by using only photoshop, or iPad - both works fine. But I find it a bit messy to manage all the files on the iPad if I am doing a book project.