That's a very materialistic view on things, and dangerously close to CGs own motives of squeezing money out of everyone. Even if there weren't rewards, it would be a fun side activity for players to participate in, unless you wanted to propose a small rewards system for the winner (though such a system would quite obviously be abused, so there's that)
Not everything has to promise some shiny reward. I still use the GCs to test out some squads because it's easy to select the difficulty level within them, or replaying journey guide battles I've completed just for the heck of it. This game needs some fun activities that don't just feel like some chore to check off the list.