If you don't have tranquility in a poker game, obviously, you lose much to opponents who do, because their perceptions and decisions will remain crisp and unfiltered, while you will be perpetually perceptually clouded by your own anxiety and fear.
Fear is a tough concept.
It keeps you focused on playing your best game. They have no fear. So they play without fear, but also without caution. Sure, they're fearless, but it's the wrong kind of fearless. So you have to care about the outcome in order to play your best game.