What are the uses of customer groups?

Customer groups are mainly used for pricing and discounts. You can configure these straight from the product itself by clicking on the Pricing tab! 

First, go to Catalog > Products from your Control Panel. Click on the Product you would like to edit and select the Special Pricing tab. From here you can add tiers and group prices to offer your customers! Be sure to save once complete. 

You can also set up Promotion codes that can only be used by logged in members of certain groups under Promotions > Coupon Codes.