O que significa “I don’t see what you want to be traipsing about after dark for,” said Marilla shortly. This is from Anne of the Green Gables. What does 'for' in this sentence mean?

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I think the meaning of the word "FOR" (it's kind of bad grammar this way) might be understood if I re-write it:
"I don't know what the reason would be, FOR YOU TO BE Traipsing around after dark"

She could also have said "I don't know WHY YOU WOULD WANT to be traipsing..."

there's a rule (I think) of... "don't end a sentence with a preposition" ("for" is a preposition)

She COULD HAVE just said it simply: "Why do you want to go out, it's DARK!" haha

it's a complicated sentence. compounded by poor grammar.. but also it's an old book...