What are lookup fields?
If you’re unfamiliar with lookup fields, Salesforce describes them perfectly in the following statement:
“Lookup fields allow you to associate two records together in a relationship. For example, a contact record includes an account lookup field that associates the contact with its account”. -Salesforce
Using the logic above, we allow you to link a record’s country, state and ZIP Code values to the Country, State, and ZIP Code objects installed with AddressTools.

Why is using a lookup field for my address block useful?
Using native Salesforce functionality, you can store additional data on the regional records installed with AddressTools, or leverage additional data points that are installed with the package.
For example, you may have country standardization configured to standardize to the full name on the account, but you can lookup the associated country record and also fetch the ISO-2 and ISO-3 value. These data points can be extremely helpful for integrations. Or why not fetch the primary county value from a US ZIP code to help with territory management?