Based on his experience and instinct, a lawyer may not want a certain juror, without however being able to identify the problem or being able to explain exactly why to the judge. Sometimes, it is something in the person's body language, or the way in which he or she answers or behaves that tells him that that person will not have the necessary attention span to follow the upcoming jury trial. That is why we use peremptory challenges.
It may happen that later, the lawyer can articulate the reason why he or she used the peremptory challenge. That said, jury selection often proceeds quite quickly, and that is one of the reasons why the peremptory challenge is useful to attorneys, especially those who only practice in the context of jury trials.