Collaboration, and the reasons for it, is sometimes as poorly understood as the term "inquiry". Collaboration does not simply mean a group of students working together and it certainly does not mean a group of students dividing a large task between them and reporting back to each other on its completion.
Collaboration is critical in developing thinking for the reason that learning to think well is a bit like learning to speak a language. Just like learning a language, you cannot learn to think well in isolation. You must do it with others to learn how to do it well. Reasoning is far better understood, in fact, as a social competence rather than an individual faculty.
Just as language is about creating shared meaning, so too is our common rationality. It is not enough that I give you my reasons as to why I think something is the case or ought to be the case. I must also make sure that those reasons are accessible to you.
I need to explain why this should matter given your circumstances, and ask that you do the same for me. We need to understand what this means for each other, because we do not all process information in the same way.
Collaboration is critical in at least five ways:
Collaborative thinking allows students to understand and develop the norms of effective thinking, just as the norms of language are understood through speaking with others.
Thinking collaboratively is a form of social cognition, in which the limits of cognition are not those of the individual mind but of the group. This can be understood in two ways. First, that social cognition helps check the biases and assumptions held by individual minds, second that the outputs of one mind can act as inputs for other minds so forming a greater cognitive complex.
Creativity is a core component of critical thinking and this is best developed collaboratively.
Feedback is a necessary condition for improving experiential knowledge (knowing how). In collaborative inquiry, in which thinking is shared and communication is clear and direct, the interactions between participants provide opportunities for immediate and frequent feedback on the appropriateness and quality of student thinking, either from their peers or from the teacher.
Collaborative inquiry both collectively forms and models the norms of effective thinking allowing development in critical thinking to be guided and shaped. The internalisation of what we learn socially becomes available as a resource for individual, private thinking.
So while it's possible for some students to develop the skills and dispositions of a critical thinker via individual learning, collaborative inquiry is the process that necessary to develop the essential skills of social cognition.