It lets you display more granular information. You can have light blue buttons with dark blue text, as well as solid blue buttons, to convey "primary" and "secondary" CTAs for instance.
You could try to display the same amount of info in less color, or even black and white (maybe outlined buttons, bold/regular fonts in the text), but color variation looks "good" to the majority of eyeballs.
You could try to display the same amount of info in less color, or even black and white (maybe outlined buttons, bold/regular fonts in the text), but color variation looks "good" to the majority of eyeballs.