When the mere silhouette of a character is identifiable, expressive and visually interesting I feel I'm on my way to a successful illustration.
Illustration is all about telling a story and it's critical that the story is conveyed clearly. By looking at the silhouette, I can assess if my gestures are jiving with the story. The witch above is clearly recoiling from the water thrown at her. Her bent posture and splayed hands project that fact. And we know she's a witch because of the pointy hat, gnarly fingers and scary chin... well, that wasn't too hard to figure out.
In fact, all these characters are pretty easy to identify–particularly that four-legged beast. Still, it's useful to consider silhouette when composing any work of visual art.