As the article notes, the spark gap is supplemental ESD protection for high voltages (above 2000V). You still need other protection for lower voltages: RC input filters, zener diodes, etc. to protect sensitive semiconductor inputs, some of which can be damaged by 10V or less.
This protection is local to the board's common return so it's not absolutely necessary to connect the board common to earth common. Sometimes this can cause ground loop noise problems. But it's generally good practice to have the chassis of your system connected to the main's earth ground (not electrical common) connection. That way if there's any fault to the chassis no one will be shocked.