When should external objects be primarily used in Salesforce?

External objects are designed to enable Salesforce to connect with and display data stored outside of Salesforce without actually storing that data within the Salesforce environment. This is particularly beneficial when dealing with a large amount of external data that you do not want to store internally. By using external objects, organizations can access and manipulate data from external sources like databases, applications, or web services in real-time without the need to duplicate that data in Salesforce. This approach helps in maintaining data integrity, reducing storage costs, and ensuring that users always work with the most up-to-date information.

The other scenarios, such as storing all data in Salesforce or managing negligible amounts of data, do not align with the specific purpose of external objects. External objects are not intended for data that will be fully managed within Salesforce or for small datasets where simpler solutions like regular custom objects could suffice. Additionally, real-time updates without storing data do not capture the primary use case of utilizing external objects, as they are specifically meant for accessing external data efficiently and maintaining that connection for up-to-date information. Therefore, using external objects is ideal when there's a need to work with extensive external data while keeping it outside the Salesforce platform.