Advantages:

Augmented safety:
Possibility of doing research which - for safety reasons - cannot be done on the road

Higher experimental control:
Driving tasks can be specified and possible disrupting factors like weather conditions can be eliminated.

Lower costs:
To adapt the environment and characteristics of a simulator is less complicated than modifying an actual vehicle.

Diversity of options:
By means of a simulator it is relatively easy to regulate different driving tasks like speed or the lateral position on the road under various research conditions (e.g. day and night, different weather or road conditions).