Typefaces set the stage for the tone and feel of a project.
Look at the sample above and imagine the type of project each font could be used for. You will have ideas and preconceived notions of what each shape makes you think of that are different from others' ideas.
Fonts set the hierarchy of text importance. See the sample below; it has the same size formatting for all aspects of the document. This makes it difficult to easily gravitate to the main ideas.
Now look below, using different sizes and typefaces makes the structure of the document easier to follow.