Many people who start training feel, that as they are doing physical activity and burning calories they can wait a while to get serious about their diet. You can go for a jog for 30 minutes and burn 300 calories. Alternatively you could have eaten 300 ham and cheese croissant last week. There is little difference.. aesthetically (forgetting fitness benefits), you are still 300 calories lighter (Roughly 40gms of bodyfat).
In a weightloss situation your calorie deficit is everything. The training you do is extremely important to keep the body and metabolism strong during dieting, but the diet itself is where you can shed the calories most.