The answer is: people conform when they face ambiguous situations
In the study, Sherif put the subjects in a dark room and told them to to watch a pinpoint of light and let the researchers know how far has the light moved.
Sherif put several actors among the subjects in order to collectively support a false interpretation and voiced their opinion to the group.
The result is fascinating. As it turned out, large portion of the non-actors subjects actually choose to disregard their opinion and choose to conform to the actors'opinion in the ambiguous situations.