Consistency is extremely important when it comes to interaction design. Consistency through a product, service or website makes it not only identifiable but has aesthetic qualities too. If a website used different colours, font types, font sizes and colloquialism throughout each page of a website, it becomes very jarring for the user resulting in a negative experience for the user.
As you can see from the images below. Google is extremely consistent with their design. They have the same theme of colours, fonts and font sizes. This not only lets users know they are using a Google product, but it connects to learnability too. Because they know it is a Google product, they will have an inclination on how to navigate each service.