Promotes diversity and social skills in meeting new people and working with those who were strangers just a little while ago. Fresh thinking with different people.
Otherwise, they clump up into their little groups that they've been in since 7th grade, because Max will do the project and everybody else can just sign off, or the special ed kids will be shunted into their own little corner, or girls and boys will self-segregate themselves.
I'm a firm believer that socialization is at least as important as any other part of the curriculum. Me assigning groups also gives cover to those who might be a little tired of Max, but don't want to say so and rock their established group.
Here's just one example from years ago when I first began. It's a sophomore English class, and the assignment is to write and illustrate a book with a moral. I've assigned the groups, and the students are getting to know each other a bit. One group, which is made up of a prominent attorney's child, a member of the school board's student, a local TV sportswriter's kid, and a wheelchair bound, cognitively impaired kid who has just transferred from another state.
They're having trouble coming up with a moral, so the lawyer is leaning into a group next door, and they're talking about cheating. The transfer student has spent many hours in hospitals and being around doctors.
Here's the story in brief:
Don and Dave are best friends, and have been since kindergarten. Don is really smart, and Dave is lazy, so Don lets Dave cheat off him, starting with fingerpaints in kindergarten, all the way through school to high school, through college, through medical school, Don does the work, and Dave cheats. One day, Don is in a bad car wreck, and as he's being rushed into surgery, the last thing he sees is Dave's face, looking down over him, saying,"Now's my chance to make up for all your work for me, buddy! I'm going to do my best in the operating room for you!" The end.
Absolutely one of my favorites to this day. Team work is effective when it is set up correctly. Just my 2 cents.