My left supraclavicular lymph node is big hard. Why only one side?

Cannot tell. A large supraclavicular node may be due to many different things, and whether it needs to be biopsied depends on whether "hard" means stony hard, whether it is fixed to underlying tissues, tender, continuing to enlarge, how long it has been there and many other things will influence decision-making. See your doctor and get this taken care of.
Monitoring Centers. Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.