Why do so many people ask for pictures on Instagram?

I know it's a scam of some sort but Im genuinely curious on why they ask for pictures, mostly if they can see how I look with a pfp or post, they just dm me, starting with a conversation sometimes and then ask for pics, what's the scam behind this idea if anyone knows, let me know, I wanna be one step ahead lol
Why do so many people ask for pictures on Instagram?
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