First, lightning rod makes Marowak immune to thunder wave. Some teams rely on thunder wave to check sweepers, so lightning rod can help a lot against those teams. Second, there are many Pokemon, such as Zapdos, mega Manectric, Magnezone, and Tapu Koko that rely on electric attacks. Marowak can only check those Pokemon if it has lightning rod. Third, even outside of those four Pokemon, electric attacks are quite common. If a Marowak had lightning rod, it could switch into all those electric attacks without taking damage. In the case of volt switch, it can switch in AND disrupt an opponent's attempt to switch in their own Pokemon. On the other hand, the only thing rock head does is prevent flare blitz recoil. Marowak is pretty easy to revenge kill, with or without the recoil. In fact, lightning rod is so much better than rock head that almost twice as many people chose to use lightning rod when compared to rock head.
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