Price Books allow you to create pricing schemes for different sales tracking codes. Instead of creating multiple catalog plans with different prices to be used, you can use a price book and a sales tracking code to easily use a catalog with the an appropriate price.
For example, you can have a subscription that you can have a price of $35.50 available for your Tampa clients and have a price of $20.50 for your Orlando Region clients on the same plan. Using the sales tracking code, the price book will determine the price to apply.