When you're going to be going to trial, one of the most important things your attorney can help with is the selection of a jury. By being present and making sure there is as little bias as possible, you get a chance at the fairest trial.
A jury must look at all aspects of a case and determine if you are or are not guilty. It is unfair to stack the jury against a person facing charges. For example, if you are accused of child abuse, then it would not be fair to allow jurors to hear the case if they have child abuse in their past. This would give them a bias, and it would be unfair to you.
Your attorney has two ways of preventing jurors from continuing and becoming part of the 12-person jury. The first is called a peremptory challenge. Each attorney gets only a few of these, but it allows the attorney to reject a juror without any particular reasoning. Another thing your attorney can do is object to a juror by challenging the juror for a cause. For instance, in an animal abuse case, your attorney would object to an animal-rights activist being present. The only thing your attorney can't do is base his or her rejection of a juror based on gender or race.
You can talk to your attorney before the jury selection, so you know what to expect from him or her during the selection process. It's absolutely vital to your case that the jury is as unbiased as possible, so you are given a fair and equal chance at trial.
Source: FindLaw, "How Are Potential Jurors Selected?," accessed March 03, 2017