Here is a nice article for Web Design patterns : http://developer.fellowshipone.com/patterns/

The primary benefit of using design patterns is that we are not constantly re-inventing the wheel when coming up with the general functionality of a site or web application. Another benefit is that these design patters often mirror or mimic other similar functionality found throughout the rest of the Internet. This allows users to feel more comfortable and familiar with a web application, rather than having to learn a foreign system.

Design patterns can help applications be understood quicker. They can be thought of in terms of various automobiles. Though they differ, the steering wheel and pedals are readily familiar to any licensed driver.