To get an idea that is in your head out of your head so that you can think of another idea.
To store an idea so that you can refer to it later.
To help resolve a partially formed idea. Ideas are often incomplete – drawing the incomplete idea out helps to resolve the unfinished parts of the idea. By putting down what you know you can fill in the blanks.
To get another idea. Once an idea is drawn its presence on paper helps to evoke another idea.
To maintain a record of ideas enabling you to understand your own idea developmental progress.