Slow repetitions are harder repetitions and better repetitions. By lifting the weight slowly, we avoid the use of momentum. When we eliminate momentum, we put the target muscle under more tension. All of the results we seek from strength training are related to the ability to place tension on the muscle.
Slow repetition speed not only produces better results, but it is far safer. The slower we go, the less force exposed to our joints and connective tissue. Moving 100 lbs at a four second pace generates about 105 lbs of force, meaning you are exposing all of the connective tissue of that movement to a force that is well within its limits. But move the same weight explosively and now you are exposing all of the connective tissue to 600 lbs - 800 lbs of force. This can exceed its limits and results in micro trauma after micro trauma until full blown injury.
Often people are concerned about injuring themselves by using a "heavy" weight; in fact, injuries occur when "forces" are high.
In order to keep the forces our joints are exposed to low and constant tension on our muscles, it is imperative that we lift and lower weight slowly.