Repetition and habits form the foundation of most human behaviours in life.
And the same can be applied to website user behaviours.
When it comes to web design, repetition can manifest in many ways. You can use repetition to form patterns which can add an appealing visual element to a design.
Alternatively you can use repetition of user actions to enforce behaviour and generate a consistent user experience.
Repetition can also help emphasise branding and design composition.
For instance always locating specific elements in the same place across different pages.
Clever use of repeating colours and typography can enable users to have a subconscious awareness of their place on your site.
In fact associating colours with services is a great way of sticking in users heads.