In the play, Twelve Angry Men, why do the jurors take a vote before any discussion of the evidence, and what is the impact of the outcome?

It's usual for a jury to vote before deliberating because
they want to know where they stand before discussing the case. It might be that all
twelve of them agree on the verdict, and then it wouldn't be necessary for them to have
to discuss the case. If the jury is split, they know they will probably have to take
several days to go into the case in detail.

After the first
round of voting, only one juror votes not guilty, and this angers some of the jurors who
just want to get out of there because it's hot and they're tired. Three of the
jurors give Juror Eight a hard time because he votes not guilty, but Juror Eight says he
can't give a boy the death penalty without at least talking about it first. The other
eleven jurors argue about it, but they finally decide to
deliberate.