Given the systems complexity of modern airliners, many are two-crew. Depending on the type rating of the aeroplane, there may be a requirement to have your IATRA. This is because, when applying for a two-crew type rating, you will need to provide proof that you obtained at least 70% on the IATRA exam within the previous two years. The same may apply when acting as a cruise relief pilot too.
IATRA is actually not a licence, permit or rating, and it does not give you any additional privileges. Rather, this 50 question 2 hour exam is focused on testing the knowledge requirements to obtain a two crew type rating - something that ACE can help you achieve.