Why must investigators collect a broad sample of hair?

Well they not only take broad samples as a part of getting evidence, but with the DNA recognition software that they use, it can help with finding out someone who may have committed a crime or was somehow involved in an incident that is already in their intel, if the person isn't already in their database, then they can keep it in there in case another event takes place and they find a sample with similar or the same DNA so that they can track or figure out a pattern of where a suspect might be going.