This is not an absolute requirement, it instead depends on the reaction. Without it, oxygen, or moisture may leak in, before refluxing goes strong, and even during. If the reactants are sensitive, they may be consumed, or produce side reactions, or fail completely.
What is your reaction, and how is it supposed to be set up, and why? These sorts of concepts are your responsibility, before you start a reaction in the university organic lab, or industrial production laboratory.
Please trouble yourself to read the
Forum Rules{click}, if you're still learning, we're glad to help. But if you want to do something at home, with minimal preparation, we don't want to give you "just enough" info, to let you hurt yourself.