Mood boards may include things that never actually show up in the project, but 

have been chosen because they represent the qualities of a brand or 

just illustrate a certain feeling or aesthetic.

Creating a mood board to present to clients gives them an idea of what the finished product will look like and allows everyone involved to agree on a direction before too much work is done. It also helps avoid any misunderstandings that may result from trying to describe a design concept verbally.