Similes and metaphors (and other imagery) can help the reader 'see' the world as the writer does. They can evoke a particular image in the reader's mind by associating one object or idea with another. A tree thin and sharp as a fork, for example, is very different tree from one that is like the fat foot of an elephant.
Similes compare two things explicitly (usually using the words "like" or "as...as"): The pine had branches like the tangled spokes of a broken bicycle. The sycamore had bark as wrinkled as an elephant's hide. Metaphors don't use those words to compare things. The comparison is implied instead: The London plane wore its camouflage jacket. The tree was undressing. Her pea-green pleated skirt had dropped to her ankles.