Using different font sizes on a single slide is a great way of building visual hierarchy.
For example look at the slide below from Paul Kemp-Robertson’s TED talk . Notice how font size is used to draw attention to the most important element of the slide:
If you pay attention to billboards and other forms of advertising, you’ll see this same principle in action:
Varying font sizes ensures that your typography is not monotonous and boring. It adds excitement and variety to your typography, making your text sexier to look at.