When you first begin to form a relationship with someone at school or work or at your local gym, small talk is the communication tool you will use to begin relating to them. Small talk acts as a sensor and actually does a lot more than people consciously realise.
Through engaging in small talk you are able to determine how interested another person is in you. If someone gives you continuous eye contact and a friendly smile you are likely to expand on your initial small talk. You started talking about the weather, now you may ask how their weekend was. This is what I like to call secondary small talk.