In/At

Hello. In my student's book I found an explanation for usage of the following prepositions: in and at. In is used to express a position inside a place. It suggests three dimentions. At is used to express a location at the point. It suggests two dimentions. And some examples are given. "He works in an office in London. I've left my case at the office. I'll see you at the cinema. She's at home. Justin's at Ben's house".
I understand everything about In, but almost nothing about At. At Ben's house means inside the house. A person has left the case inside the office, not at the office in the street.
What did I miss in the explanation?
Thank you in advance!