Question

When is estimation an effective way to determine an answer

Answer 1

If the number contains a lot of decimals, it is effective to do determine the answer.
Also, it is often useful if you work with square roots, fractions etc. As a way of checking your answer, to see if it makes sense (however, it shouldn't be used as the answer)

Answer 2

When the answer is a large number, estimation can be pretty effective.