An estimation is an effective way to determine an answer if
- the exact answer is not very important (for example it's enough to know you need 1000 grams of flower, you don't need tknow if it's 1001 or 1003)
-it's more important that the answer is fast than accurate - for example if you need to assess if 5 cups with 83 grams each will fit into a 350 grams container - the answer is "around 400" grams, and it's enough to tell you it's not enough