I wonder which one of those suggested that they include a mystic who touches herself while reciting a bastardized version of the Lord's Prayer that has no relevance to the story whatsoever.
SPOILER
-- you've been warned---
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I think it was to show just how weird and f'ed up she, and her cabal, are. It does help "dial in" on her supreme level of "crazy" mighty fast, which also immediately (and reflecting-back later) on her massive insecurities, how weak she really is while evoking strength (strength thru "epic crazy" is an inverse weakness), and ultimately what we learn is actually *jealousy*. She later states it explicitly. She's mega-jealous of the girl who gave birth to the twins, and thinks that she herself should have been given that. She wants to "have gods children" which will further validate that she herself is a god, as she has been trying to persuade her followers (that's why they live in the dam, where they can generate power "to use the light as a weapon"). She believes the light (ergo vision) is the weapon that destroyed the world before them (obviously we know this is bonkers; she *is* "epic crazy" after all).
Thus, that scene dials us all in on her "epic crazy" immediately; it's foreshadowing!
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I bet those people all were out in the woods demonstrating the slapping of whips against the forest floor as a means of getting around.
Nah, that's just standard Hazard Avoidance, plus "listening" to the reflections/echoes.