Why is teamwork important?

Answer 1

teamwork is important because you get things done faster
Teamwork is important because you and your fellow co-workers are there for one purpose is to carry out the goal(s) of the company. If there is no teamwork in the dept you are working at, then you should talk to your supervisor/manager about the person(s) and maybe those people will be disciplined or fired.

How can you work if there is tension among your coworkers?

Answer 2

This question bears some thought because the next question will surely be "give me an example where you were on such a team ?".

So think of an answer that sets you up for the next question as well.

Thus the first answer is not as important as the next answer will be. For example, answer the first question as "teamwork is very important because it improves morale / helps solve difficult problems / utilizes the power of the whole team ...".

Then, when the followup question surely comes, say "on my last / recent project / job / work assignment I was on a really great team. We did the best job of ... we had the best results ... ".

Always consider these philosophical questions an opportunity for the interviewer to followup with the next question asking for specific examples from your past work history.