Estimation is a means of simplification. It is a practical way to quickly determine the order of magnitude of a particular item, and is useful when taking an exact measure would be too complex, time-consuming or costly.
Of course, the problem with estimates is that they are by definition WRONG. And the “accuracy” of an estimate is judged by how far the estimate is from the actual, correct answer. Unfortunately, we tend to be really bad at estimating in certain circumstances, as illustrated by problems with estimates related to obesity in the U.S.