Here is my two pennies..
If you are Born and raised in English speaking country, lets say UK or USA and you claim on your application that you are excellent in English then you may not have to give a test, they will obviously realize that you have command on english. If you are from a country where the first language is not english and no matter how many years you live in a English speaking country, you have to sit in IELTS test to prove it. This also goes for people from UK or US and raised in some non english speaking country, they also need to sit in IELTS.
They don't want to see your grammer, they want to see if you understand, read write and speak english before you immigrate.