from https://www.pcgamer.com/behind-the-addictive-psychology-and-...

Overwatch's loot box is a masterpiece of audio-visual design. "It's all about building the anticipation. When the box is there you're excited at the possibilities of what could be inside," says senior game designer Jeremy Craig. Click the ‘Open loot box’ button and the box bursts open, sending four disks into the sky. Their rarity is indicated by coloured streaks to further build the suspense. "Seeing purple or gold you start to think about what specific legendary or epic you've unlocked. This all happens so fast, but it was those discrete steps that we felt maximized excitement and anticipation."