There are two aspects to this question. It asks why cooperative learning is effective and it asks why cooperative learning is essential. Let us take each of these aspects in turn.
Cooperative learning is (or at least can be) effective because it requires students to work together with one another. On any particular lesson, it is likely that different students will have different understandings of and approaches to the task at hand. It is likely that no one student will understand the task completely. Students can then help one another learn. Teaching someone else a topic, or helping them learn it, is an activity that can actually help the person doing the teaching. The person who does the teaching will come to understand the material better because they had to explain it. If each student gets to do this, all students will learn better.
Cooperative learning is essential in today’s world because many people will have to work in cooperative groups or teams when they are adults in the working world. Because of this, it is essential that students learn how to cooperate with one another. It is essential that they learn to accomplish tasks as a team instead of knowing only how to do things for themselves.
Thus, cooperative learning is both essential and effective as a teaching strategy in today’s world.