I can't speak for what country he's talking about, but that type of adapter is used in the US with older outlets that don't have the third ground pin.
In the US, homes are basically supplied from a center-tapped transformer. The center-tap is neutral, and is connected to earth ground at the entrance panel. Each side of the transformer supplies 120 volts to neutral, with high-current-draw appliances being connected cross the legs for 240 volts.
Here's a modern 120v outlet. Narrow pin is hot, wide pin is neutral and the round pin is ground.
Going back in time, polarized 2 pin outlets were used. Wide pin neutral, narrow pin hot, and possibly a ground connection to the outlet box, making the center cover plate screw ground.
Going further back, this is a non-polarized outlet with both prongs the same size. Chances are the center screw is not grounded.
The wiring for this type of outlet often was "knob and tube" which used two separate conductors with cotton insulation. The wires were run maybe 8" – 10" apart, secured periodically with knobs (ceramic insultators) and protected where they ran through studs (framing members), floors, etc. with ceramic tunes. It will be the worst day of your life if during a remodel, you open a wall and find knob and tube wiring! Your project justed turned into a total gut job.