If you work as an ALT in the public school system, whether you like it or not, you're playing custodian to the next generation. Your job may be to assist the home-room teacher teach English, but your responsibility is to help the children under your watch grow into adults who can participate in the international community they will inhabit. Teamwork is now and will be a part of that future.
Many hands make light work. Many minds can contribute different ideas and the result can be something completely different than a single person could have come up with alone. But it won't happen without great communication. This is why students need to practice working in groups, so they can develop group work skills.
More benefits of teamwork are that it can increase moral (happier students), increase confidence (more speaking in class!), develop relationships (so students can share success and build momentum but also feel failure less severely as it is shared with others).
Too Many Cooks
Too many cooks spoil the broth. Too many people working on one task can make it more difficult or more complicated than it needs to be. Another problem is when students can't handle working together. Again, this is why students need exposure to group work, so they can learn the social skills they need to participate.
Teamwork in English Class
The great thing about teamwork is it's instant communication. It may not be in English, but hey, let's work up to that.
Have you ever called on the class for answers only to have one or two raised hands from the same one or two students who answer every question every day? Well, one of the simplest ways to start with teamwork, and increase student focus and participation is to give students 10~20 seconds to discuss answers in pairs. Ask your question then let students check their answer with the student next to them. Watch as students discuss your question and find the answer together. Then see more hands raised to confidently give the answers.
One of the best thing about assigning group work is that it really pushes the focus and responsibility to the students. Suddenly, the teacher is not the center of attention and students are now looking at each other to produce.
Again, communication may be in Japanese but as long as they are producing something in English, they're getting the teamwork experience they need.
One point to note about groups is that just because students are in groups, it doesn't necessarily mean teamwork is happening. Teamwork is about solving problems together. An example of teamwork is students working on creating a dialogue together. An example of groups without teamwork is students earning points for their "team" without any interaction between member.
Teamwork for English
The final word on teamwork is that by making your classes a place where students work together and develop a comradery with their classmates, you're making it a place students want to return to. Nothing improves a student's learning like personal motivation so create successful teamwork opportunities and see your students flourish.