Diamonds are the 2nd and Most Precious Currency in game and a Basic Resource
you can gain Recharge, bonuses from Hot Events, Check-in, Lucky Tree, and Capsule Toys, FB Inbox (broken event) and Much more others
Diamonds are needed to Buy Items you really want, and they go away Fast
Suggestion: Use Diamonds for your Dailies and Unlock to have more Per Day, and Buy Items that will improve your Battle Rating, Stats, Collection