Bonding and grounding are two distinct concepts. Bonding means making two pieces of equipment with no potential between them. Grounding means each piece of equipmet has no potential between it and the ground. You could have two pieces of equipment grounded, but they may still have a potential between them. The earth under one unit may have a different resistance/capacitance or base voltage than under another piece of equipment.
This really come into play with truck loading. The tank is grounded, but the truck grounding system may not have the same grounding potential. To alleaveate the potential between the truck and tank, you bond them together with a cable.