My perception is that the difference is the message an author communicates. The message could be "I will tell you something important" or "Go buy this thing, in

My perception is that the difference is the message an author communicates. The message could be "I will tell you something important" or "Go buy this thing, in the meantime I will do some hand waving". There is no need to add a link to a book in the article which is not about this book, as much as adding a link to Github account in a blog post about unrelated stuff. Otherwise it makes a piece of writing look cheaper in my, probably wrong, view. Maybe be it's just me, seriously.