Orwell admits that he is guilty of many of these flaws that he criticizes in the this statement:
Look back through this essay, and for certain you will find that I have again and again committed the very faults I am protesting against.
Here, he not only confesses, but encourages that the reader challenge his work. Therefore, he is recognizing this as a collective problem.
However, if readers take a look at the metaphors and similes Orwell creates, they learn that these devices have difinitive intention and purpose. The difference between his and those which he cites in order to criticize is the "staleness of imagery and lack of precision." Orwell's similes and metaphors all generally serve the purpose of making his point more clear to the reader. He notes that many metaphors have endured overuse to the extent that it is now a common phrase in which all people generally understand the meaning, but there is no more excitement or vivid image created to satisfy the reader's palate.
It is also important to note that many of the poor similes and metaphors used are used to exemplify the very problems he wishes to discuss.
The purpose of a simile or a metaphor is to create a visual image for the reader. This image, once experienced helps aid understanding of a concept that may not be completely clear by just stating the concept outright.