Words ending in -ly answer the question in what way, and this one answers it with the eyes. Picture a short channel running from a thing to a pair of eyes. Stand at the eye end and you get the plain meaning: doing the job by looking — showing a diagram, checking a bridge, confirming a runway. Stand at the other end, looking back at the thing, and the same channel gives you the appearance meaning: what the eyes receive, whether it is attractive or dull. And when the channel itself is damaged, English says visually impaired.
From the word’s history
Wiktionary records visually as visual plus the adverb ending -ly, so the word literally says in a visual way. That is exactly the channel picture: -ly asks in what way, and visual answers with the eyes.
Wiktionary 記錄 visually 由 visual 加上副詞字尾 -ly 組成，字面就是「以視覺的方式」。這正好對上上面那條路的畫面：-ly 問的是「用什麼方式」，visual 回答「用眼睛」。
Wiktionary entry for visually, etymology tree: visual + -ly (packet source_ref kaikki:3f6f69a190036ae4:1311195408)
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