Usage of に in 仕上げには重宝してる

Question

「マジで人手が足りなくてさ……なっつんも中学校通い始めてから捕まりづらくなるし……ま、口は悪いけど作業は正確だから、仕上げには重宝してるよ」

The speaker wanted a person nicknamed なっつん to help her finish her work.

What would be the function of the に? Is like “use なっつん usefully for the finish”? Like AをBに使う？

Answer 1

I'm inclined to think that the use of this に is either that of "動作・作用の行われる時・場所を表す" ("indicating the time or place where some action or effect takes place") or that of "ある属性や能力を持っている対象を表わす" ("indicating an object/thing with regard to which [some entity] has a given attribute or ability").

The "動作・作用の行われる時・場所を表す" function is synonymous with "において" and comparable to "in", "during", "when". etc. in English, while the "ある属性や能力を持っている対象を表わす" one is comparable to "for" ("肌に良い" "good for skin") , "at" ("日本語に熟達している" "proficient at Japanese"), "against" ("熱に弱い" "weak against heat") , etc.

Let's see how these uses work with a couple of examples.

• この辞書は技術的な翻訳に重宝する　"This dictionary is useful in/for technical translation."

On the "time/place" reading of the に, technical translation is the (figurative) place where, or time when, the dictionary comes in handy. It's like saying "This dictionary is useful in technical translation."

On the "object" reading, technical translation is the thing for (or with regard to) which the dictionary has the attribute of "being useful". It's like saying "This dictionary is useful for technical translation."

• このサイトは日本語を学ぶのに役立つ

On the time/place reading: "This website is useful in (or when) learning Japanese."

On the "object" reading: "This website is useful for learning Japanese."

Moving onto the problem at hand:

仕上げには重宝してるよ

Since the implied subject of 重宝してる would be the person the speaker is talking about, it may be translated like this

S/he comes in very handy in finishing (the work). (on the "time/place" reading)

S/he comes in very handy for finishing (the work). (on the "object" reading)

(The two uses seem to me to sort of blur into each other. I think the two uses can be covered by "when it comes to" in English.)

Addressing the discussion in the comments, I think insofar as we consider the subject of "重宝する" to be the person the speaker is talking about, the role/capacity ("as" or "for") reading of the "に" requires one to regard that person themselves as a 仕上げ, and that seems something of an irregularity and rather improbable in the context, since "仕上げ" refers to the act of finishing, the final step, something that serves to finish some work and such, but it does not have an established, purely agentive use as meaning "one who finishes something". In the previous question, the role/capacity reading of the "に" works since what would be considered a 仕上げ is the act of putting on a "band-aid like thing", which fits the meaning of "仕上げ" well.