My guess is that the phrase is derived from the composition of the following:
役 (yaku) : service, use
に:particle for destination
立った:perfective of 立つ (tatsu)
So it's extended meaning like "rise to a supporting role", "being supported".
Am I more or less correct?
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I have read posts:
Is "役立つ" a recent lexicalization of "役に立つ"? Is it less formal?