Strong evidence proves the author's argument/claim correct. There are real examples that show that the author's claim is true. If the author chooses weak evidence, the readers will not be convinced that his/her claim is correct. If it has no relation to the claim, it seems as though there is nothing to back you up. No evidence at all is even worse, because the author isn't even trying to prove that they are correct. You have nothing to convince the reader that you are right, so they won't believe you.
