Doing a small thing extremely well is its own kind of purpose.

Purpose usually gets described in large terms, which makes most actual lives look insufficient by comparison. Depth works as well as scale does. Being genuinely good at one narrow thing, and doing it properly for the people who rely on it, is a complete answer rather than a smaller version of a real one.

Take whatever you already do often and raise the standard on it deliberately. Notice who benefits when it is done well and who suffers when it is not. That is usually enough to turn an ordinary task into something you chose.