It doesn't really have much to do with how brain works. Graphical way of arranging stuff is just something you are accustomed to. I find graphical ways of organizing information harder to use and much prefer hierarchical structures.
I don't know about Dominic System, but Memory Palace method is not about analyzing and/or grokking relations between items at all. It's simply a method that helps you remember stuff. Outlining is not about remembering stuff. It's about analyzing, breaking into component parts, understanding relations.
After outlining you will typically "remember" a subject better, but that comes about as a side-benefit of having a better understanding of the subject, not because of some method that simply helps brain recall things.
