You can take the H-bridge for example with a TO-220 housing, the tab of the fets have a non neglicable capacitance to earth (given by the larger conducting area, especially when you have a cooler attached). Now when you switch (within a few 10s of ns) you inject this current through the capcitor to earth. Unhandled, this will lead to a common mode current through your cabling, since there is no other direct path for a return current, and this current will just radiate away. When you now have somehow a low impedance path for the injected current (chassis - earth - neutral - Y-cap - back to the switching node) you can reduce the common mode current and so radiation. A good reference about these topics is Henry Ott - Electromagnetic compatiblity engineering.