The positives and negatives of lifting weights

There are a  lot of benefits to lifting weights, or any strength training.  Your muscles get stronger, and are able to do more.  It keeps you fit as you get older, and helps prevent muscle atrophy that occurs without it.  It increases your metabolism for up to 36 hours after your workout, and overall in general because increased muscles mass increases your base metabolic rate.  Muscle actively burns calories, more so than fat, or other parts of your body.

The real question is how do you get the maximum benefit from lifting weights?  That all depends on your goals.  There are programs out there like Les Mills Pump, that focus on low weight, high rep, and high calorie burn for an extended time.   That's great for getting lean and toned.   In these workouts you focus mostly on the concentric phase, or "positive" phase of the lift; that portion of the move where the muscle is actively moving the weight against gravity.   The tempo tends to be constant in say bench presses, with a 1 up, 1 down movement, though it does vary throughout the workouts, as do the range of motion.

Classic weight training or bodybuilding, such as in Body Beast, uses high weight and controlled reps.  Where a lot of people don't gain maximum results from this style of lifting is that they concentrate nearly entirely on the positive phase of the lift, and only barely control the weight on the way down.   That's old school, classic lifting.  It allows you to get more weight up, but it takes away from the time your muscle is actually being used.  That "Time under tension" has been shown to have a much more profound impact on muscle hypertrophy, or size increase, than the old school way of lifting.

So if you're looking to maximize your muscle growth, when you lift a weight, make sure the negative portion, or eccentric phase, lasts around twice as long as the concentric phase.   In other words, 1 count to squeeze the weight up (not push, sling, swing, throw, the weight up; squeeze it in a controlled movement), then a 2 count on the way down.   This drastically increases the time the muscle is under tension and thus the impact on the muscle.   It also has the added bonus of actually lowering the weight you're using to get greater benefits, which allows your form to stay more solid and helps avoid injury.   Men injure themselves in the gym all the time trying to lift heavier than they can with good form.  They sacrifice form to get the weight up and that's where injuries can occur.

So lift smart.  Good form  Concentrate on the squeeze up, and slow negative movement.  You'll see marked improvements over time.