As I have tried to show, modern writing at its worst does not consist in picking out words for the sake of their meaning and inventing images in order to make the meaning clearer. It consists in gumming together long strips of words which have already been set in order by someone else, and making the results presentable by sheer humbug. The attraction of this way of writing is that it is easy. It is easier — even quicker, once you have the habit — to say In my opinion it is not an unjustifiable assumption that than to say I think. If you use ready-made phrases, you not only don't have to hunt about for the words; you also don't have to bother with the rhythms of your sentences since these phrases are generally so arranged as to be more or less euphonious. When you are composing in a hurry — when you are dictating to a stenographer, for instance, or making a public speech — it is natural to fall into a pretentious, Latinized style. Tags like a consideration which we should do well to bear in mind or a conclusion to which all of us would readily assent will save many a sentence from coming down with a bump. By using stale metaphors, similes, and idioms, you save much mental effort, at the cost of leaving your meaning vague, not only for your reader but for yourself. This is the significance of mixed metaphors. The sole aim of a metaphor is to call up a visual image. When these images clash — as in The Fascist octopus has sung its swan song, the jackboot is thrown into the melting pot — it can be taken as certain that the writer is not seeing a mental image of the objects he is naming; in other words he is not really thinking.
--George Orwell, Politics and the English Language