When coupling a gooseneck hitch, why does the AAMVA manual instruct the driver to attach safety chains as one of the required steps?

AAMVA CDL Manual, Section 6.4.7, Step 3, p. 6-12

Safety chains are the backup that keeps a trailer connected to the tow vehicle if the primary coupler, whether a fifth wheel, pintle hook, ball, or gooseneck clamp, ever fails or separates. Treating them as a mandatory step on every trailer, not an afterthought, is what keeps a coupler failure from becoming a runaway trailer.