[Jason Durall]

Originally Posted by frogspawner

If they're both doing it, why can't they just both roll?

Two examples of why to use cooperative skills:

Increased chance of success:
For example, Susan (skill 20%) successfully aids Mathias (skill 75%) in a cooperative task. Mathias now has an effective skill of 85%. This raises his chance of a special success from 15% to 17%, his critical chance increases from 04% to 05%, and his fumble chance drops from 99-00% to just 00%.

Indirect assistance:
Mathias (Pilot 15%) is on the ground, in an air traffic control tower. Susan (Pilot 25%) is at the controls of an airplane, trying to land it after the pilot was shot in a terrorist attack. Over the radio, Mathias offers advice to Susan on the situation, including giving her what advice he can based on weather conditions and an outside appraisal of her approach. He makes a successful Pilot roll, raising Susan's Pilot effective skill from 25% to 35%. She is immensely grateful for this increased chance to land the plane successfully.