(And to be clear, none of these involve sitting down and writing some generic Tree<T> structure, they're all cases of "Well, there's some tree-like datastructure that already exists or is a natural fit for this system and I'm going to be traversing it depth-first or breadth-first to do something to the elements of it or calculate something based on what nodes I pass through on a given traversal")
- Every html document is a tree structure. And css documents have special syntax to address nodes within those trees
- If it's any good, you're routing framework probably uses some kind of tree to quickly match the request to it's handler
- The database you write to uses a tree to quickly find the location it needs to write to