The vast majority of jury trials are held in criminal cases. The other most popular use of juries is in accident and other injury cases. It is my opinion that juries were guaranteed for criminal cases to protect citizens from their government should it attempt to prosecute them for political crimes or prosecute them just because it didn't care for them. Given our experience with and separation from England our founders drafted our Constitution primarily to protect us from the government. I doubt that our founders were overly concerned about violent offenders.
It is unclear where juries came from or why they are traditionally made up of 12 people. Our US Supreme Court said many years ago that a 12 person jury is an "historical accident." Some attribute the number to the disciples and others to King Henry II who selected a group of 12 landowners to resolve boundary disputes among other things. The bottom line is that over time different cultures decided that a group of people might be better at resolving certain conflicts than an individual.
Juries are very unpredictable, which is why they are used mostly in criminal cases and personal injury cases where plaintiffs are seeking huge amounts of money. In criminal cases a defendant can "win" when the jury can't reach an agreement because the case will have to be tried again, which could lead to a better plea bargain at least. A second mistrial could even cause a prosecutor not to try a third time because of the time and expense involved. Attorneys also hope in criminal cases that the jury will find the defendant not guilty because it likes the defendant or dislikes the law involved or the government's behavior. Juries are told they can't do this but will do it anyway. I'm sure you can remember a case where the defendant was clearly guilty but still was found not guilty by the jury.
In personal injury cases involving serious injuries and death attorneys hope the jury will award amounts that a judge would not even consider. This is especially true when the defendant is a high profile company with deep pockets. It is hoped that the jury, out of sadness, concern, or dislike for the defendant, will base its decision on emotion rather than reason and award the plaintiff an astronomical sum. Admittedly, it is difficult to place a dollar amount on not being able to walk again because of an accident, or losing a loved one, but there is a general belief in the legal community that juries will award more money and that that possibility encourages defendants to offer a larger settlement.
I had a trial professor in law school that specialized in criminal jury trials. When discussing juries he told us about a trial where an attractive young female juror in the front row hung on his every word and smiled at everything he said. In the back row was an older gentleman that scowled at him the entire time. He lost the trial and learned afterwards that the scowler was his greatest supporter and that the cutie sank him. Such is the personality of the jury.
You can't even request a jury in civil suits when you are seeking something other than money, such as in a divorce or mortgage foreclosure. In most civil actions where you can request a jury it is my opinion that the judge will do a better job. Also, a judge trial is generally less time consuming and less expensive. However, if your case is assigned to a judge with a known bias or prejudice that may affect your case then opting for a jury makes sense.
When I taught trial law I didn't spend much time on jury selection other than to tell my students that if your client was an X you wanted X's on the jury and if your client was an O you wanted O's on the jury. Trial lawyers have all kinds of ideas about what type of jurors are best such as young people are more tolerant of crime and poor people will award larger verdicts against rich folks. If we learn anything as we grow older it is that you don't really know people or what they will do based upon the external categories they fall into. The personality of every jury is different just like the personalities of its members. You have heard about cases where jury consultants have been paid extraordinary amounts of money to help attorneys select jurors. You could spend millions of dollars and still not know if the jury you are facing is the Lady or the Tiger.