The fight or flight response can help to keep you safe from a threatening situation. When you have a threat, such as someone chasing you down a dark street, you have an automatic response to the danger, known as the fight or flight response. So a part of your brain called the amygdala senses the danger, and the amygdala in turn activates both the sympathetic nervous system and the HPA axis. This cascade then leads to sympathetic nerve stimulation and the release of adrenaline, a stress hormone released from the adrenal glands.
The end result of the activation of the fight or flight response in the face of danger is that your body is preparing almost instantly to either fight the threat, or to run from it. So your body is preparing to protect itself by making you stronger, faster, and more focused. And you don't even need to think about it, as it is an automatic response, a reflex that occurs almost instantaneously.