You can think of filler as fill plus -er: the thing that does the filling. First there is a gap — a crack in a wall, a hollow under the skin, half a page left over, a silence in the middle of a sentence. Then something goes in and closes it. The supplied etymology cuts the word exactly that way, into fill and -er, and that line is still visible in every use. One more thing worth noticing: what goes in is chosen because it fits the hole, not because it is the best thing available. That is why, when filler is used about songs, pages or episodes, it carries a shrug with it.
From the word’s history
The packet's etymology tree for filler ends in English fill + -er, with fill going back through Old English fyllan to a Germanic root meaning full. So the word is built to name whatever makes something full, which is exactly the core image.
來源資料的字源樹把 filler 收在 English fill ＋ -er，而 fill 再往前經過古英語 fyllan，回到日耳曼語表示「滿」的字根。所以這個字從構造上就是在指「讓東西變滿的那一樣」，跟核心畫面完全對得上。
Wiktionary (kaikki) etymology tree for filler, as supplied in the evidence packet: Proto-Germanic *fullaz > Old English fyllan > Middle English fillen > English fill, plus the agent suffix -er.
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