Permission sets in Salesforce are used to grant additional permissions to users beyond what is provided by their profiles. Permission sets can be used in a variety of situations, such as:

Granting additional permissions: When a user needs to perform a specific action or access a particular feature that is not enabled by their profile, a permission set can be created to grant them those additional permissions.

Providing temporary access: Sometimes a user needs access to a feature or data for a limited time. In such cases, a permission set can be created that grants them temporary access to the required feature or data, and then the permission set can be removed once the access is no longer needed.

Restricting access: Conversely, permission sets can also be used to restrict access to certain features or data. For example, a permission set can be created that prohibits a user from viewing or editing certain fields or records.

Providing access to managed packages: If a managed package is installed in Salesforce, a permission set can be created to grant users access to the package’s features and data.

In summary, permission sets are used in Salesforce to grant additional permissions to users beyond what is provided by their profiles. They are useful in a variety of situations, including granting temporary access, restricting access, and providing access to managed packages.