Marker is mark with -er on the end, and -er names the thing that does the job. So the whole word can be read as: the thing that leaves a mark. Hold that picture — something is left behind on purpose, and later somebody reads it. The thick pen is the plainest version, because you can watch the mark appear. From there the mark moves off the paper. It becomes a post in a field, a pin on a map, a number in your blood, a promise made in public, a favour remembered. The material changes every time; the job never does. Something is left where it can be found again, and it stands for something that is no longer in front of your eyes.
From the word’s history
The evidence packet gives the formation directly: marker is mark + -er, where -er goes back through Old English -ere to a Germanic agent ending, itself shaped by Latin -arius. So the word has always named a doer of marking, and every sense here is a different kind of doer.
證據裡直接給了構詞：marker 就是 mark 加上 -er，而 -er 這個字尾經古英文 -ere 一路上溯到日耳曼語的施事字尾。也就是說，這個字從一開始就是在指「做出記號的那一方」，底下每一個意思，只是換了一種做記號的方式。
Wiktionary etymology tree supplied in this evidence packet (source_ref kaikki:3f6f69a190036ae4:1101821535:49382): From mark + -er; -er via Middle English -ere, Old English -ere, Proto-West Germanic *-ārī.
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