The human eye requires contrasts for visibility and legibility.
Contrast creates visual interest and helps deliver accurate information.
For example, black text on a dark blue background is hard to read. When the color value is too close between text and background colors, it can create legibility problems. To identify close color values, squint at the chart below. The letters that disappear into the background have color values similar to the background. When you are putting together different elements, you want to be sure they have a contrasting color value unless you want the elements to just blur together.