There is little competition for that space. The Replace the car with walking and riding a bike. There are times when walking isn't practical. However, there is nothing keeping me from just taking a 10 minute walk before starting a session. Calories are a measurement of the amount of energy that comes from food.
Exercise requires energy.
The intensity of the exercise increases the number of calories burned. Walking quickly burns more calories than walking slowly. Your body weight also changes the amount of energy burned. It requires more energy to carry a body that weighs 240 pounds up ten flights of stairs than to carry a body that weighs 120 pounds. You may be familiar with tables that tell you how many calories are burned doing specific activities for a specific amount of time.