The author may want to make it look like they have authority when in fact they do not. The second possible motive is to sway the opinion of the reader. Usually, this will be accomplished by using sensational language. If the author is writing a persuasive essay, he may employ the use of overgeneralization to sway the opinion of the reader. Take a look at this example right here of overgeneralization. “Everybody knows she is a terrible teacher.”
Here, the author makes an assumption that cannot really be believed. It may be that most people do indeed have a negative view of the teacher, but to say that everybody feels that way is an exaggeration. Here, the author is claiming consensus when none actually exists. When a reader spots an overgeneralization like this one, they should become skeptical about the argument that is being made.
An author will often try to hide a weak or unsupported assertion behind authoritative language. If you see a statement like this and you can recognize it as an overgeneralization, know that that means the author is probably trying to hide a weak or unsupported assertion behind authoritative language.