Looks like the author only uses one session he calles "work". What I really like about sessions is that it allows for isolated contexts which are easy to switch (prefix-s get prints you a nice list [1]).
Right now I have seven sessions open [1]. Three are work projects (they usually have windows for vim, db console, etc.), two are university courses (vim, matlab), in one session I'm learning some Haskell, and in the last one I'm working on an open source ruby gem. If I had all these windows in one session only, I'd constantly be switching to the wrong windows.