When you use your muscles you use the energy stored in them (glycogen). In order to replace that energy (which your body does continuously every time you move, lift, whatever) you need calories. Your body will either use what is in your system (excess blood glucose) or, if there isn't any excess, will access your fat stores to replenish your muscles.

In addition it costs more calories for your body to maintain muscle than for it to maintain fat (the magic of fat, from an physiological POV, is that it's easy to store, provides insulation, and costs little from the body to maintain, all of which is v. handy if you're about to face a long lean winter in an unheated cave). So the more you lift the higher muscle mass you will have (women tend to find it very hard to "bulk up" because growing bigger muscles depends rather heavily on androgens which women have in sparse supply, so you end up toned and strong rather than bulky) and the higher muscle mass you have the more calories you can eat without gaining weight.

Resistance exercise is what your body is built to do and it's extremely good for you. It's great for your muscles, your bone density, the cardiovascular systems and it lowers stress levels.

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