In some places in Canada there's many airports that are served by literally only two airlines. Air Canada and Westjet. It's certainly possible to use a 3rd party booking site to try to find a 'cheaper' price to go to/from those places, but as you'll see when you get your search results for a flight, there's basically no competition. Go directly to the Air Canada or Westjet websites themselves and the prices will usually be the same or slightly better.