You learned how to flatten trees into arrays using Euler tour. Every subtree becomes a range [tin[v], tout[v]], turning subtree queries into range queries.
This enables segment trees and Fenwick trees on subtrees. Updates to a node affect one position. Queries over a subtree become range queries.
The technique appears in problems combining trees with range data structures. Once you see "subtree sum" or "subtree update," think Euler tour. It is the bridge between tree structure and array-based data structures.