In Salesforce Sales Cloud, when working with Sales Territories, a territory model must be activated to view any reassigned accounts. This is because only an active territory model applies the defined territory account assignment rules to assign accounts. While a model is in the Planning state, it allows for setting up and testing, but changes are not applied to live data. Activation finalizes the model and makes the assignments visible and enforceable. This ensures that accounts are correctly reassigned based on the updated rules in the territory model.
For further information, refer to Salesforce documentation on Activating Territory Models.