One action holds this word together: a person walks straight in. Nothing stops them and nothing has to be arranged first. Sometimes what makes it possible is size, because the space is big enough to stand in. Sometimes it is the doorway, because there is no high edge to climb over. Sometimes it is the rule, because nobody has to book a time. And sometimes it is simply the way you arrived, on foot instead of in a car.
From the word’s history
Wiktionary records walk-in as deverbal from the phrasal verb walk in, which is why every use stays close to that action.
維基詞典把 walk-in 記為 deverbal from walk in，也就是直接由動詞片語 walk in（走進去）轉成的詞。這解釋了為什麼它的每一種用法都離不開「走得進去」這個動作。
Wiktionary entry for walk-in (etymology_text: 'Deverbal from walk in.'), supplied in the evidence packet
登入就可以看每一支怎麼連回核心畫面。