Q1: "Why? They've sat there through the whole trail, so why not discuss it with each other jury members."
A1: Let's suppose that Alf and Mary are both members of the jury. They have a quiet drink together at the hotel. Alf states why he thinks that the defendant is, in his opinion, guilty. Mary listens and finds herself agreeing. By the end of the conversation, they've both become convinced of the defendant's guilt and they're unlikely to change their minds.
Now let's suppose that, instead of having that conversation over a drink, they have it in the jury room. Alf puts forward his arguments. Mary is about to agree but another juror, Dave, says "Hang on a minute, what about the evidence of Mr Bloggs?". Ethel now joins in: "Yes, I think that's very important". Now Alf and Mary are forced to listen to counter-arguments before reaching their conclusions. i.e. it's of paramount importance that EVERY juror must listen to the views of ALL the other jurors before reaching their decision.
Q2: "Also, when the jury are trying to reach a verdict in the court building then whole else is present, other than the 12 jury members? And if there is someone what is there role?"
A2: Jurors always discuss a case in private. However, they're able to call upon the services of a court official who can, for example, pass a note to the judge seeking legal guidance for the jury. Normally, however, the sole function of the court official is to inform the judge when the jury are ready to return to the courtroom.
Chris