In life, Kiowa made a considerable impact on those around him. A Native American Baptist, he was able to combine Christianity with his indigenous heritage to provide a unique perspective on the Vietnam War in which he and his platoon are fighting.
Due to his considerable wisdom, Kiowa is able to get his army buddies to talk about their experiences, most of which are, as one would imagine, thoroughly unpleasant. The other men feel able to talk to Kiowa because he is empathetic and compassionate. They can unburden themselves to him in a way that they can't with anyone else. It's certainly difficult to imagine O'Brien talking to any other member of the platoon about the North Vietnamese soldier he's just killed.
In death, Kiowa's impact is no less considerable. His manner of death—drowning in the filth of a sewage field—perfectly, if graphically, illustrates the true nature of war: brutal, dirty, and hellish.
What happened to Kiowa could just as easily have happened to his buddies in the platoon; if Kiowa hadn't died in this way, someone else almost certainly would have. His sudden, tragic, and gruesome demise therefore stands as an uncomfortable but important reminder to the others that a similarly undignified end potentially awaits them round every corner.