Contrast is produced when two or more visual elements in a composition are different. It can be used to create specific effects, emphasise the significance of certain elements, and add visual appeal to your designs.
Designs that look the same are boring—by experimenting with contrasting colour hues, shapes, sizes, textures and typography, you can liven things up. Humans tend to like contrast. It’s a great way to grab attention, control the visual flow and keep folks engaged.
Keep in mind that adding too many variations can be confusing for viewers (the opposite effect you want to have.) As with most of the different elements of art, it’s about striking a balance.