For one, you don't have to have the firm amount of travelers or even their names until X days prior to the trip. If you decided to skip over that and do it in advance of the required deadline then changes/ref fees start once tickets are issued. So if granny isn't a sure thing on the family trip to Vancouver you can hold her seat/fare without paying for it.

But again, I don't know how it works for groups contracted outside of Canada with AC