The best possible situation for teamwork is when each member of the team has complete confidence in the intentions and abilities of the other team members, as well as a clear vision of the goal and the steps that lead to it.
Perfect teamwork is invisible. In such an optimal situation everything goes on smoothly and efficiently, and potential problems are taken care of before they even arise.
Of course, in real life people are rarely competent, understanding, trusting, or tolerant enough for such a level of teamwork.
Communication keeps the operation of the team well-oiled and without friction. With a smaller group of people teamwork consists of recognizing the problems as they arise, successfully communicating with the other members of the team, and making the necessary adjustments.
As the teams get bigger and the goals more complex, communication becomes harder to accomplish. To be able to function at this level, the team has to be well-structured. It becomes necessary not only to communicate, but to do so through properly defined channels, for clear reasons, and with appropriate level of priority.
Good teamwork maximizes the effect of communication with minimal overhead. (If it's the other way around, it's called 'bureaucracy'.)