If two objects are grounded, why do they still need to be bonded? And if they are bonded, why does each still need to be grounded? (fire safety)
They need to be grounded to equalize the voltage between the container and the ground to prevent static discharge. The grounding wire also provided a current pathway to ground to provide a pathway in the event of static discharge

They need to be bonded to equalize the voltage between the source container and and the container to be filled and to provide a current pathway to ground 

The other nice thing about grounding and bonding is the electrostatic discharge would go on the outside of the container, and then through the wire. 

If there wasn't a grounding or bonding wire, the pathway could potentially be through the flammable liquid (if conductive) or through the hose, thus creating sparks near the nozzle and air inside the container that is most likely saturated with gas from the liquid that is being poured. You need both for a current pathway. I wouldn't see much difference if you grounded both containers instead of bonding one to the other.


https://www.ccohs.ca/oshanswers/prevention/flammable_static.html