Suppose you are hiring an Accountant. A critical soft skill for an Accountant is "Integrity" and (I hope) all successful Accountants have this ability. However, we know we are not all created equal when it comes to soft skills. Some have stronger standards than others and if we didn't test the candidates "Integrity", they would all simply pass through. Setting the Percent of People to Pass value ensures you are qualifying candidates who have a higher standard on this and other important soft skills.
Realize that leaving this setting at 50% does NOT mean that exactly 20 out of 40 people will pass when they are assessed. It is an approximate setting against the population applying.
There is another situation to consider. Suppose you know you are hiring for a position with a smaller candidate pool. In that situation, you can raise this number to ensure you get a healthy flow in your hiring funnel. On the other hand, if you have a very large candidate pool for a position, you may consider lowering this number to get candidates with higher levels of these soft skills.