Collecting 'Free case' and 'Mission case' by completing the missions guarantees fixed supply of in-game currency which helps in upgrading/purchasing weapons and armors. Make sure to collect and complete it daily.
Free case can be collected after every 6 hours.
Mission case can be collected after completing the mission on reaching Daily points 30, 60, 90, 120 and Weekly points 1000 and 1500.
New set of mission are assigned to the player after every 24 hours.