When we talk about functions we are looking at specific language tasks. They can be related to students’ jobs or personal lives. At work they may need language for answering a phone or sending an email. Outside of work they may want to make a hotel or restaurant reservation. Many parts of daily conversations can be broken down into functions as well. Things like greetings or showing thanks are very helpful in daily speech.
One nice thing about teaching from a functional point of view is that it’s easy to manage. Students see a clear progression of what they can’t do at the start and what they can do at the end. Ideally every class allows students to express themselves more clearly. Functional language study is one way to make that happen.