Make us feel
Good typography is emotional. The choice of typeface affects how your content is understood or perceived. Based on its character, the right typeface sets the tone and voice for your text.
Make us read
Good typography is utilitarian. It allows the reader to focus on the content, not the formatting. Ideally it is not even noticed because it just makes sense to the reader.
By using guidelines and rules, the audience can determine the most important points of your presentation just by looking at it. This makes it easier for your audience to follow along and pay more attention.