Why do we have adrenaline in our system?

Very important. It's your fight or flight hormone. It affects almost all parts of the body. It works differently in different parts of the body but in general it is responsible for getting you ready to run away or fight the immediate danger: increase breathing, increase heart rate, dilate your eyes so you can see better, break down fat to get energy, stimulate production of sugar for energy, contract muscles...
Fight or Flight. Adrenaline is one of the main hormones of the "fight or flight" response, which is ingrained in us as humans. It helps feel alert, and it raises heart rate and blood pressure - in an anticipation to run from danger. It is a very important hormone.