The idea of RTFM is that if you have questions, then we are all on same page with basic information found in the manual. I mean you expect others explain what is already said in the manual. Its like asking how to use your microwave oven, even if you have the manual right at your hand. Now, if the manual is unclear or difficult to understand, that is a different story. Then you can at least say you didn’t understand it. The point is, that you did something before (your homework) and looked at the obvious places like the manual (and maybe further websearch).

People don’t like others being lazy and asking the questions that doesn’t need to be asked. That’s why RTFM exist. As much as you might take the “RTFM” as an offending answer, those people think of you question as offending too. Now there are people who use this term loosely in places when it is not appropriate. Also it depends on the audience. If your grandma tries to use a browser to watch funny cat videos, and asks how to use it, then it would be inappropriate to say RTFM. But if you have a Linux user who asks about how to use grep, then I think it is an appropriate reply.