Goundig probes are used to eliminate the possibility of stray voltage to enter the tank and ultimately effecting the livestock. Be it through your powerheads and even the lighting system to mention a few. The best way to think of it is a bird landing on a powerline, the pole is grounded to the earth whereas, if there is no grounding ptobe in a tank the voltage has no place to go and the tank itself acts as a ground.
The probes themselves are relatively cheap ($20+-) for the protection of your inhabitants and most importantly whomever puts their hands in the tanks water. I hope this helps and Welcome to Reef Central. Tinman