Suppose you move to Canada. Now suppose you have rich relatives who want to come visit you often - birthdays, christmas, vacations, etc. for them, it makes more sense to get a multiple entry visa.
Another - admitedly unlikely - situation: you take an "american continent" tour: canada, then down to the US, Mexico, Guatemala, Panama, then all the way back up to Canada - you get a multiple entry visa, because you'll be entering canada twice.
A third situation: you are a foreign student in Canada, but you miss home so you go there for a week or so once every four months. You should get a multiple entry visa.