I disagree on both the specific and the general points.
General: The idea of separating presentation from content is useful in some contexts (including much of the web) where you need to be able to pour the content into multiple different containers. In other contexts, particularly the design of physical objects, presentation and content should be tightly connected. (You use the example of a booklet, but a booklet should be designed in a way that's well adapted to its use, and its use will depend on its content. There's no one-size-fits-all ideal booklet design.)
Specific: Even if you do want to learn to better separate content from presentation, learning the box model, or the different kinds of positioning, won't help in any but the narrowest way. There's no reason to learn CSS unless you're designing for a context that renders CSS. CSS has too many intricacies and perversions to be valuable in the abstract.
General: The idea of separating presentation from content is useful in some contexts (including much of the web) where you need to be able to pour the content into multiple different containers. In other contexts, particularly the design of physical objects, presentation and content should be tightly connected. (You use the example of a booklet, but a booklet should be designed in a way that's well adapted to its use, and its use will depend on its content. There's no one-size-fits-all ideal booklet design.)
Specific: Even if you do want to learn to better separate content from presentation, learning the box model, or the different kinds of positioning, won't help in any but the narrowest way. There's no reason to learn CSS unless you're designing for a context that renders CSS. CSS has too many intricacies and perversions to be valuable in the abstract.