Do I need a biopsy if my lymph node is big and sore?

Doubtful. How long has it been that way, is it fixed to underlying tissue or mobile, is continuing to enlarge, where is it located, do you know what may have caused it to enlarge? Watch this for a while. If it does not begin to resolve, seek physician input.
Maybe... All cases of lymphadenopathy are different. If you are concerned, please see you doctor for evaluation. In general, painful lymph nodes tend to be related to infections or autoimmune disorders. Most physicians attempt a trial of antibiotics before sending patients for a biopsy.
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.