With the stardust reward it helps trading. Makes gym turn over happen more. Brings reason to fight a gym even after you get your daily coins.
I love gym battling but i want more incentives, with this implemented we do just that.
edit - Also this forces you to go to new gyms for the better rewards of dust. Remember if you go to the same gym the dust amount decreases. Would be a challenge to get double 5 hours after your 50 coins!