Icons are a simple, easily graspable way to add visual emphasis or interest, signal an action, or indicate a feedback state — all while reducing a user’s cognitive load.
Using a consistent set of icons helps establish a familiar look and feel across a site or service, while using individual icons consistently establishes a reliable relationship between the image and a specific concept or action. For example, don’t use an envelope icon to indicate email on one page and mailing address on another. Users should be able to trust that a certain icon always means the same thing, no matter where it’s used on your site. It’s important that icons always serve a single functional purpose throughout a site or service.