Who has a grounding probe in their tank?

[0sprey]

They complete the circuit- so if there is stray voltage, it flows through your tank. You stay safe, because you can't ground the tank yourself anymore, but the current still flows through the water. It's not as dangerous to fish as it is to humans, but it causes their navigation systems to go berserk. Messes around with the sensory pits in the lateral line, if I recollect correctly.
With a GFI, on the other hand, as soon as leaking electricity is detected, it shuts down the circuit. No damage. Of course, then you have to start testing to figure out which piece of equipment is leaking... which could be a PITA. But still better than taking a risk, IMO.