That is certainly one definition and use case of heuristics. Another example might be the travelling salesman problem, where we know how to calculate the exact

That is certainly one definition and use case of heuristics. Another example might be the travelling salesman problem, where we know how to calculate the exact solution for every instance, but it is prohibitively expensive (unless P=NP). There are a number of heuristics that "can find solutions for extremely large problems (millions of cities) within a reasonable time which are with a high probability just 2–3% away from the optimal solution" (Source: https://en.wikipedia.org/wiki/Travelling_salesman_problem https://en.wikipedia.org/wiki/Travelling_salesman_problem) In that case, you can always come up with a solution, it's just not always the best solution.