Knowing and understanding the intended audience is crucial for the author of a technical document because they're trying to capture the focus of this audience. Thusly, you would want to use words and whatnot that appeals to said audience. For example, you would not want to use malodorous words such as splendiferous while trying to capture the attention of an audience of little kids. This is because they would not understand the word's meaning and thus would not pay much attention. Additionally, you would want to make a theme that captures the attention of the audience as well; for example, teenagers may be attracted to themes with music involved. Without understanding and knowing the audience, an author cannot properly capture their attention nor convey their meaning effectively.