In many countries, cable broadband is carried in underground conduits that are less likely to be hit by lightning. Some cable broadband is provided (at least partly) over optical cable which is not conductive and does therefore not transmit an electrical surge. Phone-line broadband can be supplied on phone lines that are suspended from telegraph poles above the ground, and are much more likely to be hit by lightning.
In general, every conductive cable that enters your computer is a potential path for a destructive surge and might benefit from surge protection. This could include Ethernet cables, serial cables or perhaps USB cables that link to a device that has a separate connection to an unprotected electricity supply, etc.
an average bolt of positive lightning (from the top of a thunderstorm) may carry a current of 300 to 500 kiloamperes, transfer a charge of up to 300 coulombs, have a potential difference up to 1 gigavolt (a billion volts),