you skim whenever you want to get some general ideas about and article or a book. For example, when you
-want to find out about a recent election. There is a long magazine article on the subject, but you don't have time to read it all.
-need some information about the theories of Sigmund Freud. You have found many book about Freud,and now you need to know which one will be most useful.
-went to a movie and you though it was terrible. You notice a review of that movie in today's newspaper. You want to find out quickly if the reviewer agrees with you.
-are reading a detective story and you are in a hurry to know who did the crime.