There's a reason why every exercise you perform comes with instructions. They're not just to make you look good in the weight room—they're to train specific muscles. To get the most out of each exercise, it's essential to adhere strictly to the instructions. This will ensure that you safely gain the full benefits of your strength training program.
One primary reason why technique falters is using too much weight. Take the Bicep Curl for example. Some people curl a large weight for show, but they bring their elbows forward and lean back, which reduces the effectiveness of the bicep-isolating exercise. For optimal results, less weight should be used so perfect form can be maintained—with the bicep actually doing all of the work.
This philosophy of "less is more" must be applied across all strength exercises. There's little sense in performing an exercise—such as a Back Squat—if you aren't able to successfully go through the full range of motion, or if you must compensate for a weakness. You will work harder with less weight, fully developing your muscles through the prescribed range of motion and using proper technique to produce on-field improvements.
Finally, poor or improper exercise form can increase the risk of weight room injuries. Lifting weight places enormous stress on your body, and exercise technique is specifically designed so that you can move the weight safely. If your technique falters, you place vulnerable parts of your body, such as your back, shoulders or knees, in a danger zone, where they are subject to various injuries that can diminish your strength training and degrade your competitive play.