Groupthink is a phenomenon in which people try hard to maintain unanimity instead of trying to think of alternative ways in which to solve a given problem. Groupthink is often seen as a bad thing, but there are some circumstances in which it can be good.
The most obvious example of a time when groupthink could be beneficial is during times of crisis. You can imagine how this would be true of a group like an army. When an army is engaged in a battle, you want everyone thinking in the same way. You do not want to have separate units engaging in different tactics because their leaders did not engage in groupthink. The same general idea would apply to a sports team during a critical moment in a game.
Groupthink can also be good in situations where cohesiveness is seen as the most important goal. Our typical image of Japanese society is like this. In Japanese society, people engaging in groupthink are more likely to be socially successful. This may or may not be beneficial for the society as a whole, but it is clearly beneficial for the people who engage in groupthink.