A good ground is important for surge protection devices (SPD) to work properly. AC mains SPD's are designed to divert surge current to ground by providing the least resistive path. Without surge protection on the AC mains, the surge current will look for other paths to a good ground. In many cases this path is found through electric/electronic equipment. Once the dielectric strength of the components in electronic equipment has been surpassed large currents begin to flow through the sensitive electronics thus causing failure.
There are not many areas of the world today that do not experience surge-related incidents. Lightning is only one of the many causes of transient surge related problems. Today's modern electronic equipment is much smaller, much faster, and much more susceptible to transient related problems than was the last generation of equipment. The shear number of control and communication devices interlinked together in today's networks make their susceptibility many times greater. These are new problems that were not nearly as frequent with previous generations of control equipment.
Full protection is one of the most inexpensive insurance policies you can buy. The cost of system in-availability is far more expensive than proper protection. One major surge event in a ten-year period far outweighs the cost of protection.
The protection the Phone Company provides is there mainly for personal safety to prevent lightning from migrating in on their wires and causing personal injury. It provides little protection for sensitive electronic communications equipment. It provides primary protection but not eliminate the need for secondary protection at the equipment.
Basically, this is a device that sits between your PC or other electronic equipment and the electrical mains (AC) outlet (ie, wall plug) and protects your equipment's power supply (and possibly communications lines) from electrical surges. Any power from the mains must pass through the surge protector to reach your gear. A surge protector regulates the current to connected equipment by either blocking or shorting to ground any "unsafe" voltage. Usually set up in a "strip" or box form, surge protectors have several power outlets to plug into. Better versions will also have at least one data outlet to protect a telephone, fax machine or modem as these are also in danger from a nasty surge of electricity through a phone line.