By invoking Crocker's Rules, you allow other people to optimize their communication with you for a maximum flow of information. Thus, you free them from the strict norms of society concerning meaningless fluff, politeness and all the stuff your parents told you about being nice to people.
With textual communication being already so limited in bandwidth compared to face-to-face interaction, this is supposedly a way to let other people communicate more efficiently with you.
The problem are people stating the rule and believing they're now allowed to say whatever they want whomever they are talking to. You can't make someone else not take offense, it's only possible to promise that you're not going to.
Named after Lee Daniel Crocker, and quoted mostly by transhumanists believing that they are standing above such worldly concerns as emotions, it's just another way to express the robustness principle: Be conservative in what you do, be liberal in what you accept from others.
It would be a great world if more people would follow this basic idea. Wait, I think there was already a guy some two thousand years ago with a similiar idea. And we know what happened to him.