Of all the visual principles, contrast does the most work while getting the least credit. Color theory and typography get the conference talks; contrast just quietly determines whether anyone can use your interface at all.
At its core, contrast is about difference that carries meaning. The eye is drawn to whatever stands apart from its surroundings, so contrast is how you tell someone where to look first, second, and third. A primary button that's the same weight as everything around it isn't a primary button — it's just another element in the noise. Make it bolder, brighter, or larger, and you've created a visual hierarchy without writing a single instruction.
Contrast operates across more than color. Size separates a headline from body text. Weight distinguishes a label from a value. Spacing groups related things and divides unrelated ones. Even motion is a form of contrast — a single moving element on a still screen captures attention instantly, which is exactly why it should be used sparingly.
Then there's the accessibility dimension, which isn't optional. Text that fails contrast ratios is unreadable for a meaningful slice of your users, and "it looks fine on my monitor" is not a standard. The established ratios — 4.5:1 for normal text, 3:1 for large — exist because legibility was tested, not guessed.
The discipline is restraint. Contrast loses its power when everything competes for attention; if every element shouts, the user hears silence. Pick what matters most, make it unmistakable, and let everything else recede.