Knowing your basal metabolic rate is important, to keep our bodies alive even when we are at rest our bodies require a certain amount of energy (calories) per day to stay alive.
When we exercise, we go on to burn even more calories, so knowing our energy cost per day we know how much energy (fuel) we need to put into our body. If we are looking for performance workouts or if we are looking to lose weight, we need to know how many calories of energy intake we need to efficiently be losing weight, so knowing your basal metabolic rate is extremely important.
By filling out this calculator https://www.mydr.com.au/tools/basal-energy-calculator/ with your details and your activity level you will get two numbers. The number on the left which tells you your energy cost at rest and the number on the right, which is the energy you need to exercise at your desired level. To effectively lose weight, it is recommended to cut those numbers in half and find the middle and that is the energy or calorie intake you can have per day, which will ensure you can lose weight each week. If you are not looking to lose weight and looking for energy for exercise performance, you focus on the number on the right and that is the energy that you need to perform your workouts at high intensity.