The most obvious is to break an uncomfortable silence. Another reason is simply to fill time. That is why it is so common to make small talk when you are waiting for something. Some people make small talk in order to be polite. You may not feel like chatting with anyone at a party, but it is rude to just sit in a corner by yourself. After someone introduces you to another person, you do not know anything about them, so in order to show a polite interest in getting to know them better, you have to start with some small talk.