The picture behind all three uses is a habit — something done so many times that it stopped feeling like a decision. English then points that habit in two directions. Point it backwards and you get the past habit: he used to walk to school, and the walking has stopped. Point it at the person and you get the result the habit leaves behind: she is used to the noise, so the noise no longer bothers her. One word, two directions: the habit that has ended, and the habit that has settled in.
From used, the past of use in an older sense meaning to be in the habit of, plus to. The habit is in the word itself, which is why both branches are about habit rather than about using anything.
來源可以想成動詞 use 的一個老意思：「習慣做某事」。used 是它的過去形式，後面加上 to。習慣這件事本來就在字裡面，所以兩個分支講的都是習慣，跟「使用」沒有關係。
Wiktionary entries for 'used to' (packet etymology_text, source_refs kaikki:3f6f69a190036ae4:976892220 and :976902883)
One branch
以前的習慣和以前的樣子
登入就可以看這一支怎麼連回核心畫面。
did often in the past · aux · A2
You say that something used to happen when it happened again and again, or all the time, in the past. Saying it this way also tells the listener that it does not happen any more.
以前常常（現在不了）