Teamwork

One of the main reasons here is convenience. There is little value in rolling for every member of a team individually.

Doing something together is always easier than doing it alone. If you have a pip in an action that another person is doing, you may assist them. You may gain 1 stress for 1 additional dice in the ability that they are rolling.

When doing something together as a team you have a big advantage. The dice pool for the entire team is equal to the pool of the member with the highest value in the tested quality.