Lifting weights causes micro-tears in muscle fibers. As an adaptive mechanism, your body rebuilds those muscle fibers slightly larger so they can lift the weight without tearing. If you increase the weight you are lifting periodically, your body is forced to adapt again and rebuild the muscle fibers even larger. This is called progressive resistance weight training.
Having more muscle mass on your body increases your metabolic rate because muscles are constantly burning calories, even when you’re resting (as opposed to fat cells, which just sit there). Increased muscle mass also causes your body to increase your bone density to support the extra muscle. Together, more muscle mass and higher bone density make you less prone to injury. Men start losing 10% of their muscle mass every decade starting in their 30’s, so it’s important for all men to do some weight training once they reach a certain age just to maintain their natural muscle mass.
Depending on the kind of weight training you do, you can direct your muscle-building toward specific goals, such as building strength, increasing endurance, targeting specific muscles, etc. Nearly all professional athletes engage in weight training in order to improve their performance in their particular sport.