Would like to understand the importance of weight training? Why cardio alone is not enough in reducing fat? Pointers towards the above would be greatly appreciated
With fat loss as a goal, weight training is generally not the solution. And to be honest, neither is cardio.
Fat loss is achieved by regulating your diet. Spending 1000kcal on cardio can take hours, but saving 1000kcal in the kitchen is done in two minutes.
The training you do simply dictates how your body is going to adapt to the changes.
Read some answers from related questions:
The importance of weight training as it pertains to fat loss specifically depends on your goals. Weight training, when done properly, will add muscle mass. Increased muscle mass equates to a higher metabolic rate, i.e a faster metabolism. This makes losing fat and keeping it off a lot easier, i.e you can eat more calories relatively, you have more room. Vice versa, when you're in a caloric deficit in order to lose fat, you will undoubtedly lose some muscle in the process. When you lose muscle, your metabolism also slows down and hence fat burning will become harder and harder over time. In order to minimize muscle loss during a fat loss program, you need to signal the muscles that you still need them so your body doesn't burn them for energy and this is achieved through strength training. You will also improve your insulin sensitivity which basically dictates how efficiently you use carbohydrates. A higher insulin sensitivity means the person is more likely to store the carbs as fat and hence you can see how weight training comes in here.
Cardio alone IS enough for fat loss, and moreover, you dont even need cardio to lose fat. You just need to have a very good diet, and to be exact, a caloric deficit. But cardio and weight training are tools that help make this process much quicker and easier. Changing your lifestyle is a big part of this.
Exercise has been traditionally misunderstood as the way to reduce fat. While you can induce a caloric deficit by burning more calories than you take in and thus lose weight ("The Thermodynamics Diet"), it will likely be temporary and hunger will encourage you to return to a calorie-neutral or positive diet. "Calories-in vs. Calories-out", though simple and true in a physics sense, completely ignores the endocrine system with its appetite-regulating hormones and multiple metabolic pathways.
Much more effective to adjust the macronutrients of the diet, reducing the carbohydrates that increase blood sugar and insulin secretion, because insulin inhibits the utilization of stored fat for energy. Can't burn much fat when on a blood sugar roller coaster and hungry every couple of hours.
That said, weight-training is an ideal stimulus to the body that more muscle is needed. Muscle is metabolically expensive, so consumes calories even when not in use, and when used hard burns through a lot of glucose, improving insulin sensitivity and blood sugar levels. It can be a very time-efficient way to maintain general fitness and increase physical resiliency.
Cardio, while fun, is mostly good for conditioning the body for, well, more cardio. Medium-intensity activity utilizes the aerobic pathway in muscles, which only slowly depletes glycogen stores and glucose. Weight training (or high intensity interval training) utilizes the anaerobic pathway that blows through glucose at ~20x the rate. While most forms of exercise are good for increasing levels of "fitness", cardio's applicability to weight loss can be assessed by surveying those who daily use the treadmill at your local gym. Some will be slim, some will not, more or less independent of number of hours spent.
There is a book called "Body By Science" that discusses a more researched and nuanced view of exercise than is traditionally taught. It recommends surprisingly short but very intense intermittent sessions of resistance training-- a stimulus designed to provoke the desired response.
Cardio is all you need to lose weight. You must build up enough cardio fitness to be able to run fast and long enough, but one hour of running can burn 1000 Kcal or more. Burning the same amount of energy from lifting weights would require you to lift a weight of 100 kg over a total height of 1 km. So, you'll need to lift a heavy weight on a training machine an incredible number of times before you come close to what you can burn in just an hour of running.
You can also think of it this way. The only way your excess fat can leave your body is by burning it with oxygen and converting it to CO2 and H2O. The CO2 can only be lost by exhaling it, while the H2O can also be lost as urine. In total about 84% of the lost fat mass will end up being exhaled in the form of CO2 gas, so the vast majority of your excess fat will have to leave your body via your lungs.
If you do an hour of running you'll breath a lot faster during that hour than when you do strength exercises, also you'll keep on breathing at a faster rate for quite a while after the exertion.