Apps informing the user they are doing to ask for permission is not a dark pattern. It’s used to give user more context. Maybe you’ve seen it done poorly but when done correctly it’s good.
Why would you ever need that? I’m pretty sure you can already supply a custom explanation in the system permission modal to explain why you want the permission.
Because the native request prompt contains a very short string. You may use a permission for a number of things and need more description length to explain it. See the Apple HIG for an example: