Answer:
When you don't add direct input from your writing. If your first paragraph is about "how dogs save the day", then in your thesis, you should put "...how dogs save the day...". By adding input from for body paragraphs, you are informing the reader about the rest of your writing. If you don't add these key items, your thesis may be weaker, causing the reader to be less interested.
Have a nice day! :)