Not all airlines have websites that allow combining multiple airlines. This requires issuing the ticket through an OTA.
Example: I had an itinerary TP:LIS-LHR//AC:-YYZ-YOW//UA:YOW-IAD-LIS. Not a mistake fare. Pulled it up on Matrix. Only Priceline would price it properly. AC issued the ticket. Any changes would have had to have gone thru PL.