You can think of a handler as the one who takes hold of something and keeps it under control. Handle means to get your hands on a thing and deal with it; -er turns that into the one who does it. What sits in the hands changes from use to use. It can be a suitcase or a tray of food. It can be a working dog on a lead. It can be a famous person's day, or a spy's instructions. In computing it is not even a thing you can touch — just an event arriving, and a piece of code ready to catch it. Whatever it is, someone or something has it in hand.
From the word’s history
From Middle English handler, handiller: handle plus -er, the one who handles.
來自中古英語 handler、handiller，也就是 handle 加上 -er：動手處理的那個人。這個字面意思到今天還完全站得住。
Wiktionary entry for handler, packet source_ref kaikki:3f6f69a190036ae4:1434369744 (etymology_text: From Middle English handler, handiller, equivalent to handle + -er).
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