Well, a world that lacked rationality might be one in which all the events were a sequence of non-sequiters. A car drives down the street. Then dissappears. We are in a movie theater with a tyrannosaurus. Now we are a snail on the moon. Then there's just this poster of rocks. Then I can't remember what sight was like, but there's jazz music. Now I fondly remember fighting in world war 2, while evading the Empire with Hans solo. Oh! I think I might be boiling water, but with a sense of smell somehow.... that's a poor job of describing it -- too much familiar stuff - but you get the idea. If there was no connection between one state of affairs and the next, talking about what strategy to take might be impossible, or a brief possibility that then dissappears when you forget what you are doing and you're back in the movie theater again with the tyrannosaurus. If 'you' is even a meaningful way to describe a brief moment of awareness bubbling into being in that universe. Then again, if at any moment 'you' happen to exist and 'you' happen to understand what rationality means- I guess now that I think about it, if there is any situation where you can understand what the word rationality means, its probably one in which it exists (howevery briefly) and is potentially helpful to you, even if there is little useful to do about whatever situation you are in, there might be some useful thing to do about the troubling thoughts in your mind.