Picture something that is already finished, with one more piece hung on the side of it. The main thing was there first and works on its own; the extra piece arrives later, is usually paid for on its own, and can be taken off again without breaking anything. An extra cover on an insurance policy, an extra drive on a computer, an extra tool inside a program — the shape is the same every time: one main thing, plus one piece.
From the word’s history
Wiktionary records add-on as a deverbal noun from the verb 'add on'. The action of adding something on is turned into the thing that gets added, which is why the word always implies that something else was there first.
來源資料記 add-on 是從動詞片語 add on 名詞化而來（deverbal）。「把它加上去」這個動作被變成名詞之後，指的就是被加上去的那一塊。這也是為什麼這個字一定預設有個更早就存在的主體——沒有主體，就沒有東西可以加。
Wiktionary entry 'add-on' (kaikki source_ref 3f6f69a190036ae4:994540172), etymology_text: 'Deverbal from add on.'
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