Chemotherapy is in case some cancer cells are elsewhere in your body which is possible because cancer cells were in some of your lymph nodes. The chemo is designed to kill them, if they exist. If I was very old I probably wouldn’t bother. But I was 60 so it was worthwhile doing, I thought. Unpleasant, yes, but it doesn’t last for ever (although it can feel like it at the time.) The multipronged approach of surgery, radiation, chemo and hormone therapy if appropriate is why breast cancer has such a high survival rate.