A simile does more than announce a shared property. The speaker aims to clarify, establish, or make a particular idea about the subject effective. We must therefore identify both the respect of similarity and its rhetorical aim.
Common aims directed toward the subject are:
1. Showing possibility of something surprising.
2. Explaining its condition.
3. Indicating the degree of a quality.
4. Confirming or fixing the quality in the hearer’s mind.
5. Embellishing or disparaging the subject.
6. Presenting it in an unusual, striking form.
More than one aim can coexist; context determines the dominant one.