So you can get a basic idea of what your actual answer will be in range of.
Example:
10.5+5.5=__
I can safely assume with estimating that it'll be 15.
Now when I actually find the correct answer which is 16, its a safe bet knowing that your estimate is close to your actual answer.
If you make an uneducated guess like saying 10.5+5.5 was 30 then your answer was 16 its makes you wonder how you got an estimate of almost 2x of 16.
Estimates help, so you know what range your real answer would be in.
Hope this helps