We want to feel assured that we are convicting the right person, the one who did it.
We want the innocent to be freed and exonerated (which are not the same thing).
We want to correctly identify the deceased, to find some measure of justice for victims, and to prevent those who harmed them from harming others.
Because we never want to be mistaken in these essentials of justice, we want to believe that our methods of evidence examination are going to deliver the truth. If we have doubts here and there, most of us feel sure that modern science has found at least one method that is perfect: DNA.