My general understanding of why you bulk and cut is because muscles take a lot of calories (carbs and protein) to grow and sustain growth. So in your bulk cycle you focus your workouts on pure muscle and strength growth and eat a lot which causes you to both have sustainable muscle growth as well as gain fat from the excess of calories you eat. Then in your cutting cycle you try to lose the weight you gained but when you are on a calorie deficit you also lose muscle mass. So, you focus your diet to hold as much muscle as you can while also focusing your exercises on high cardio. Again this is just my general understanding of why you do it I may have gotten some things wrong