Most of us have been brought up to think of daydreaming as a wasteful activity.
Daydreaming can be wasteful, when it is too random, the main focus of one’s life, or used to indulge in negative or counterproductive thoughts.
However, a certain amount of what people commonly call daydreaming is a necessary part of the writing process.
No one sits down at a keyboard cold with an inexhaustible supply of ideas. Ideas have to come from somewhere; and they usually come from time spent in reflection. (Ideas also come from doing extensive reading of both fiction and nonfiction, and having a full and varied intellectual life—but this is a separate topic.)
The trick is to engage in directed daydreaming—otherwise known as brainstorming. Brainstorming is daydreaming with a purpose in mind.
Brainstorming is a necessary part not only of writing, but of any creative endeavor. Brainstorming is no less necessary for the entrepreneur or the corporate manager than it is for the writer.