Presets cut down the amount of time you spend editing. Instead of manually adding and adjusting settings in your photo editing software, use a preset to get you 80-90% of the way there. After that, all you need to do is fine-tune your image and export!
Once you've applied a preset from other users, you get a glimpse at the way they edit their photos. By dissecting their settings, you'll learn more about your software and how to achieve similar styles on your own.
If you're editing a series of images, you'll probably want them to look and feel the same throughout. The quickest and easiest way to do this is by using a preset. Apply a preset across all of them, and you've nailed a consistent style.
If you've hit a creative block with your editing, use presets to jumpstart those creative juices. By applying various presets to an image, you'll see possibilities and discover which direction you want to take your image.