Why is groupthink bad for a team?

Specifically, an individual with a stronger personality, may try to take over the show, another might not want to speek outloud, and proclam a good idea , because he may fear others will belittle a simple direct idea. A person s bias's against group think, are related to their past experiences with how well the team was managed, or left undirected, with minimal ground limits. Such as, each person takes a turn, and no bullying of ideas and presentations allowed.