It is always important to focus on form during the entire movement of every exercise. However, many people focus only on the concentric lifting portion of an exercise (standing up from the bottom of a squat or lifting the weight up to your shoulders during a biceps curl). Although the concentric is important, the eccentric lowering portion of the exercise is just as important, and some may argue that it is MORE important. Many articles have shown that the eccentric phase of an exercise can produce more force when compared to the concentric phase of the same exercise. So when done properly, an exercise performed with the same weight, the eccentric lowering phase of an exercise may actually produce more strength gains and muscle growth.
Here is an article from Dr. Johnathan Mike that goes into more detail on the benefits of eccentric techniques and exercises.