What is the purpose of grounding and bonding cables checked as special-purpose equipment on some tank vehicles?

AAMVA CDL Manual Section 8.1.2, p.8-8-1 (lists grounding and bonding cables as special-purpose equipment to inspect); 49 CFR 177.837(b)-(c) (bonding/grounding to neutralize static charges before and during transfer of Class 3 flammable liquids)

Static electricity sparks are a real ignition source during the transfer of flammable liquids, so a grounding and bonding cable that doesn't work removes a key fire-prevention safeguard. A driver who understands why this equipment matters is more likely to actually test it, not just glance at it.