Most students are engaged by the process of brainstorming, and are comfortable with small-group problem solving. This method allows them to see how other groups are doing. If they start to compare their output with that of their peers, they might be encouraged to put in some extra energy. However, the point is not to win the race. It might be beneficial, too, to have groups tour the posters and see what other groups are thinking.
Students are also comfortable with refining their data.
Whether the students report once or twice, verbally or in writing, the information they need to rely on is all around them and in their own handwriting. The instructor can see which students contribute to the group effort and which stare out the window. This method also provides opportunities for aiding skill development. If a group is having a hard time with their thesis statement, the instructor can immediately see the material they're working with and deduce whether it's a lack of evidence that's the problem, or whether they need additional thesis models. The method can help move a class away from simply trying to absorb content, and it demands an active classroom.