Language is the most important social tool. Without language, we can't express our thoughts, convey our feelings and exchange information. So for babies, the process of mastering language is the process of knowing the world and learning how to communicate with people around them. Therefore, language is also a very important social skill that babies need to master.
Then "learning to speak" is not only a process in which a baby never speaks until he is 2 years old, but also a process in which a child is 0-5 years old.