First of all i would like to mention that group discussion is not the process of selection its a process of elimination. 100,200,300...students sit for placement, so it is not possible to take interview of each and every candidates. and also in 8-12 min of GD you are able to check too many qualities like:
Communication skills
Body language
How you behave in a group?
How you convince other?
Are you listening to others or just putting up your points?
Leadership skill etc