Once a Freshservice instance has gone live and is in production, changes may want to occur. Depending on the change (new Workflows, installation of apps, retrieving/updating data via the API, etc.), it'll likely want to be tested first. Doing this on a live/production environment is not the best option as it can lead to undesirable results depending on the action occurring.
Providing access to a Sandbox instance would allow administrators to perform their testing before implementing the changes to their Live/Production instance. In addition to this, it will allow for the creation of test tickets without affecting the quality of the data in the live/production environment (which is important for management when running reports).