Characters are very important for the look and feel of a story. It doesn't matter if it is a picture book, animated short or a commercial.
The characters have to fit into (or even strengthen) the concept of the story in form, but also in material and style. What do they need conceptually and what can be left out? Do they look silly or heroic? Are eyes or a nose contributing to the character?
All kinds of questions are asked during the design of a character. Here are some examples of the final designs for different projects.