Think of the moment something hidden or lost comes into view, and someone or something is standing at that moment. Finder names whoever or whatever was there when the thing was found: the person whose hand closes on a dropped wallet, or the small part of a camera that brings the thing you are hunting for into the frame.
From the word’s history
Wiktionary gives finder as Middle English findere, by surface analysis find + -er — the one who finds. Every use here is that same role, handed to a person or to a part of a machine.
Wiktionary 記 finder 來自中古英文 findere，拆開看就是 find 加 -er：找到的那一個。這裡的每個用法，都只是把這個角色交給人或交給零件。
Wiktionary entry for finder, etymology field: "From Middle English findere. By surface analysis, find + -er." (kaikki:3f6f69a190036ae4:1145445459:4474)
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