Topics are a way to interact with other people on all the levels you have mentioned. A team will use Topics as a replacement for meetings, and they will make decisions faster. This is the department type of topic.
Management oversees company productivity by following multiple Topics, in which they can adjust timing, decision making process and resources involved.
Another usage scenario is to invite a customer to a Topic chat and solve their support problem on the spot, by direct communication.
Remote teams use Topics to communicate on project evolution and to exchange large files.
People create Topics for free time activities as well, communicating with colleagues and friends who share their passions/interests.
Hope this gives you a few ideas :)