An ordinary literal word can convey its conventional sense immediately. A simile invites the audience to identify a shared attribute. Kināyah may direct attention to a quality through something regularly associated with it.
Consider:
“He is generous” directly names the quality.
“He gives like the rain” highlights wide-reaching beneficence by analogy.
“His door is always open to those in need” can suggest generosity through an associated practice.
The last sentence does not prove generosity in every imaginable context; its surface meaning may also be intended. Thus the route and strength of indication depend on context, not simply on substituting one expression for another.