What are two reasons why teamwork and collaboration among co-workers produces better results than isolated individuals?

Case Study: Your company`s executive team has asked you to check in on a fellow manager the same rank as you, whose decisions seem surprising and strange. Since Katherine was hired last month to manage her division, she has broken up all the teams, divided tasks and responsibilities by individual, and ordered them to work with as little contact among each other as possible. She explains her decision by describing her previous workplace situation: Katherine was supposed to work in collaboration with her team members, but they all abused her or neglected their jobs. Her male team lead was often cruel to her and dumped too much extra work on her. One teammate spent most of the time working on her own freelance contracts. No one knew how to meet the goals upper management set for them, and they all focussed more of their energy in blaming teammates for their problems.

Question: Given her experience, what is one reason why you think Katherine broke up the teams in her division? What are two reasons why teamwork and collaboration among co-workers produces better results than isolated individuals? What are three causes for the team breakdown Katherine experienced at her old workplace?