Some agreements you have with publishers or federations may require you to trace a login back to a user for a period of time after it has happened. This cannot be done if the account has been deleted so you may need to set the automatic deletion period to be long enough to allow for any agreements of this type.
The domain administrator may have set a preference to delete non-activated accounts after a number of days. You should bear this in mind when changing the activation code expiry preference.
Resource access section
Permissive and restrictive mode for federated resources