The NFL has better training programs for officials and the CFL wants to take advantage of that. For The NFL the benefit is getting officials more time at live games before the NFL season starts. Pretty good article here http://www.thespec.com/sports-story/6510680-watch-for-nfl-officials-on-cfl-fields/
"The most notable element of the new arrangement will see NFL officials working pre-season and regular season CFL games starting this season. And while there is the perception in some quarters that American officials are better than their Canadian counterparts, it's unlikely that one NFL official on a crew of seven is going to make any appreciable difference.
In reality, an American presence on Canadian fields was the price the CFL had to pay in the quid-pro-quo with the NFL. Senior vice-president of officiating Dean Blandino has also taken some heat for the quality of his crews the last few seasons, and getting his guys more live-action training opportunities was the main attraction for the U.S. league.
For the CFL, the real benefit comes in being able to send Canadian officials to the United States for both training — the NFL has a much more extensive and well-funded program in that regard — and experience. Both leagues need reps: the NFL officials will get them in-game at the CFL level, while the CFL officials will work NFL training camps."