Proper form is essential to provide maximal stimulation to muscle fibers. When improper form is used, stress is transferred to the tendons and joints. This means that we should be focusing on more than simply moving the weight from point A to B. Small changes in the way an exercise is done can make a big difference in the results we see later.
Let's take the chest for example.
Straight bar bench press is the most commonly used exercise for chest, but far from the most effective. Yes, you can load the weight up, but often times your stronger side takes the brunt of the work causing imbalances. On a straight bar bench press, the hands are fixed in place. Keeping in mind that one of the main functions of the chest is to bring the arm across the body, relying on the bar can sabotage our gains.
I vastly prefer a dumbbell bench press since it allows us to bring our arms out wide at the bottom, while also squeezing them together at the top. Other varitions, like the hex press, make dumbbells even more versatile. A hex press keeps both dumbells pressed together above the chest allowing us to squeeze throughout the entire repetition. The more you can work on the mind-muscle connection, the better your results will be. On every rep, the goal is to feel the contraction and force the muscle to do the maximum amount of work.
You can strengthen the mind-muscle connection in a few different ways. One of my favorite is simply to use a lighter weight and focus on squeezing as hard as humanly possible. Other effective methods include Swiss ball exercises that reinforce proper form, such as the ball wall squat. Posing can be quite good as well. Another option would be static holds, where you sit in the bottom of a repetition. Holding the weight in the least advantageous position increases contraction strength, control over the weight, as well as time under tension which is a main contributor to muscular growth and adaptation.