Estimation is useful to quickly compute a value without needing a calculator. This way you can get a sense of scale of a project. Of course, when it comes time to doing the actual project, you should compute more accurate values to avoid rounding error.
---------------
For example:
2.3*5.7 estimates to 2*5 = 10
The actual value is 2.3*5.7 = 13.11; we see the estimate 10 is an underestimate. The estimate isn't too far off, so we get a sense of the size/scale of this product.