Surprised not many mentioned World Exploration
You're right. I knew I was forgoting something and that's was it. World Exploration is great. Easy to do, very nice rewards (high ampules, petite centies and free 5* and even 6* if you are extremly lucky). And it's a good reason to get more 6* or RG and train them. Most of the gameplay needs only one or two teams of 6* making all the other totally useless and benched for ever. At least World Exploration gave them a better purpose than being beautiful in the collection.