Simile is a type (or kind) of one of the four master tropes, or figures of speech, employed in poetry. The four categories of master tropes are metaphor (simile is a kind of metaphor), metonymy, synecdoche and irony.
In order for poetry to be effective, it must compress ideas and it must establish comparisons or analogies (comparison and analogy note the similarities between things). Rhetorician Kenneth Burke is the one who most famously defined and explained the four master tropes:
- Metaphor: Gives perspective through comparison or analogy between two unlike things to make unknown qualities knowable (metaphor and simile are the two kinds of metaphoric trope; simile uses a word such as like or as but metaphor does not).
- Metonymy: Provides reduction through substitution of an attribute for the actual thing meant, e.g., substituting Crown for the ruling Queen of the Royal House of Windsor.
- Synecdoche: Offers representation by substituting a part of something for the whole of something, e.g., hands for sailor or farm laborer.
- Irony: Reveals dialectic by substituting the truth with its contradictory opposite so the message heard is the antithesis of the idea thought or emotion felt, e.g., "Thanks for your help in finding my glasses" said while being handed a pair of glasses by the person who had just sat on and broke them.
These four tropes, which are figures of speech expressing meaning that is figurative and not literal, are the major means for compressing poetry into its most concise and impactful form. To understand compression, try to paraphrase into prose a sonnet written by Spenser of Shakespeare. Your prose will probably be at least three times longer than their poetry, which is compressed.
Since simile is a type of metaphor, and since metaphor is integral to composing compressed, powerful poetry, and since poetry is founded upon comparison, that is upon an underlying metaphor, it is absolutely essential that sometimes metaphors used be of the second type; it is essential that metaphors sometimes be similes. (Don't let the double use of metaphor confuse you; it's common for categories of things to have types, such as rainbow trout having several types.)