In each website, the objectif is to create a relationship. A way to do this is to give as much freedom as possible to the customer.
For instance, you can offer the customer the possibility to personnalize the environment of the website as Yahoo does.Thus he might feel more involved.
The second interest is for your company, they can know the customers' pratices and understand who they are. With these informations you can target your action and be most pertinent in your propositions.