Surge Protection Device vs. Grounding in Transmitters

I'm a bit confuse about differences between surge protection device and grounding in electronic transmitters.
As a general rule, housing of all instruments shall be grounded. Also as requested by our Client, all transmitters (and F&G detectors) shall be equipped with SPD (either internal or external).
About internal overvoltage caused by switching, I'm OK about what SPD does but when it comes to lightning, I cannot distinguish between tasks of SPD and grounding of instrument housing itself.
It would be highly appreciated if someone can tell me the difference.