Euler tour converts subtree problems into range problems. Any subtree query becomes a range query on [tin[v], tout[v]]. This works for sums, min/max, XOR, or any operation a segment tree can handle.
The tree structure is irrelevant after flattening. Time complexity drops from per query to per query. This pattern appears in many tree problems. If you see a problem asking for repeated subtree queries with point updates, Euler tour is almost always the right approach.