To pass the test with lower emissions. This means lower taxes in some countries, and good PR in every one. Edit: to expand this, consider that in order to reduce emissions like NOx, you often must sacrifice performance, so the customer will complain. When standarised tests were introduced, a crude counter strategy was to trade performance for lower emissions for X amount of minutes (the duration of the test). So customers noticed "hey, my engine works much better after 15 min!" :P