Arsenic (V) Reduction to Arsenic (III) by Microorganisms
You can find a great and recent review on this topic, which is thankfully also free to access, here: https://www.sciencedirect.com/science/article/pii/S2319417016000159

Arsenic resistance is ubiquitous among bacteria, they virtually all maintain the ars operon. Among Arsenic-resistant bacteria, this is not an advantage, but Arsenic was and remains a ubiquituous substance and being resistant to it confers an advantage over those that aren't; which is why bacteria that save themselves the effort aren't abundant.

The core to your answer is in the very last summary paragraph:


  ArsM transforms inorganic arsenic into a highly toxic organoarsenical species that kills off by competing the bacterial species and may also be responsible for carcinogenesis in animals. Competing microbial species have responded to this environmental pressure by evolving detoxification mechanisms for MAs(III). Some produces ArsI that demethylates MAs(III) to less toxic inorganic As(III) while other produces ArsH that oxidizes MAs(III) to nontoxic pentavalent MAs(V). It is likely that all of these processes are taking place in environmental microbial communities as bacteria, archaea, fungi, and protozoans constantly fight for dominance.


Summarised in Figure 5: https://ars.els-cdn.com/content/image/1-s2.0-S2319417016000159-gr5.jpg

The reason why ArsC and other arsenate reductases (reducing Ars(V) to the more toxic Ars(III)) evolved in the first place, setting off this whole Arsenic methylation cycle, is that 


  they are required for resistance to arsenate, which became the predominant arsenic species after oxygen appeared in the atmosphere.


Simply put: If I can make a poison that kills you but not me, why shouldn't I make it?