This will produce a much more realistic drive, as most people would only tow a trailer if they are going to load it with something, rather than just towing it empty. A fully loaded trailer can increase acceleration and braking times significantly, especially when carrying livestock or other delicate items that cannot afford to be "thrown around" the trailer. As a result, the driver would need to be looking much, much further along the road to allow more time to complete any task.
More details can be found at:
https://www.gov.uk/new-laden-testing-rules-for-driving-test-vehicles