Also known as Von Restorff effect. It predicts that when multiple similar objects are present, the one that differs from the rest is most likely to be remembered.

This is the main reason why all call-to-actions (CTAs) look different from the rest of the action buttons on a site or application. We want users to be able to differentiate between a simple action button and a CTA, in order for them to have a clear understanding what the CTA does, whilst also remembering it throughout their use of the application or site.