GFIs became the law of the land about 20 years ago in the USA thru the NEC. There are lots of unprotected sockets out there. Regular circuit breakers trip if overloaded or shorted but offer no protection from ground faults or most electrocutions. GFIs are essentially sensors that look for imbalances between the line (power) and the load (neutral). When there is an imbalance,like me getting zapped between my toaster and the faucet, the line shows power out but the load sees nothing. Hence , we get a tripped GFI. This can happen also if a person gets between the load and ground. It will not protect you from getting directly in line between line and load because it sees no imbalance.