When someone confesses to a crime, it is inherently more believable than someone that tells a story where they are innocent. It is a question of motivation to lie. People are not generally motivated to lie when the lie implicates themselves or their interests. It is the same rationale as that which allows a statement made by an unavailable witness to be admissible hearsay evidence in Federal Courts as “a statement against interest”. These type of statements are inherently more trustworthy.