Well under the original Repository they were one way of further defining items and here's an example:

Suppose you're an autodealer and using the Repository to show the vehicles you have for sale. So you have a category called Cars and want to allow potential buyers to further define the options. So you create an attribute for Colors and enter Red, Green, Blue for the values. Now your visitors can now sort by car color. You can also add a Model attribute (Coupe, Sedan, Luxury, etc) and then a visitor whose looking for a green sedan can easily see if you have any in stock.