What you may be talking about is that your body requires resistance exercise to maintain your lean body mass (muscle), especially if your calorie intake is below maintenance levels (ie. you're losing weight.) Since your lean body mass determines your daily calorie needs, just doing cardio exercise and/or calorie reduction risks losing lean body mass, thus making it harder to lose weight.
Note the 'just' - the key is that you need to do resistance exercise - not that you don't need to do cardio. Cardio is the best exercise for burning calories directly and it builds up the stamina levels necessary for intense resistance exercise. As the name 'cardio' suggests, it's good for the health of your cardiovascular system (heart and bloodflow). It's also good for your health in general. Cardio fitness enables your blood to supply oxygen to whereever your body needs it.
In short, if you want to lose fat you need to do a mix of cardio and resistance exercise. You can lose weight just through dieting, but that weight will be mostly water and muscle, not fat. And losing that muscle will make it increasingly hard to keep the weight off.
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