A binder can be pictured as whatever does the binding: the thing, or the person, whose whole job is to take pieces that would otherwise come apart and hold them as one. First there is something loose — a stack of printed pages, papers in a drawer, a bowl of dry , a deal that is not signed yet. Then the binder arrives and ties it down, and what was scattered behaves as a single object. The material changes from sense to sense; the job never does.
From the word’s history
Wiktionary derives binder from Middle English byndere, i.e. bind plus the agent ending -er: literally 'one that binds'. That literal reading still covers every modern use.
Wiktionary 記錄 binder 來自中古英語 byndere，也就是 bind（綁）加上 -er（做這件事的人或東西），字面就是「綁東西的那個」。這條字源和今天的每一個用法都對得上，不必另外編故事。
Wiktionary, entry 'binder' (noun), etymology supplied verbatim in this packet: 'From Middle English byndere; equivalent to bind + -er.' (source_ref kaikki:3f6f69a190036ae4:1227464479)
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