It's really hard to do (at all, let alone well) but the point is to encode more information in the problems such that, by solving them oneself, the student gain

It's really hard to do (at all, let alone well) but the point is to encode more information in the problems such that, by solving them oneself, the student gains direct understanding of some technique or application or extension that wasn't covered in the main text. In a (well-written) textbook there might be five or tens times as much information latent in the (solving of the) exercises than in the entire preceding chapter. Writing these kinds of sets of exercises is very challenging, much harder than just writing a textbook.