rationalisation

How much of the design should be rationalised to be acceptable to the audience (or the client). How do you measure that rationalisation? If I just make up a story about the design, which was created purely without a rational reason, would that be regarded as not a valid rationalisation?
It seems that a lot of graphic designs are created through this kind of process. A collection of odd random images and messages that might be put together in a certain way. The story behind it is whatever story that the creator tells to people. It seems graphic designers have more freedom to not to rationalise their designs when compared to industrial designers. But aren’t they designers, too? Aren’t we? Why should there be difference?
Maybe, it is a sort of design process that industrial designers go through as well. Who knows how our good ol’ Starky came up with the citreous juicer? Maybe he doesn’t have any rationalisation behind this. Perhaps he just woke up to it.....