There have been many studies on the benefits of exercise for lymphdema and cancer rehabilitation. Exercise get the heart rate up and in doing so, it moves oxygen into the body. Cancer does not thrive in oxygen.
Flexibility is important, especially after cancer surgeries. Gentle movements minimize the build up of scar tissue, decrease the risk of frozen shoulder (i.e. breast cancer), and reduce the risk of lymphedema (both upper and lower extremities) by improving the flow of lymph fluid.
Plus, exercise to music and being part of a group is fun. There can be lots of laughter at a time when life is threatening. Additional benefits are improved self-esteem, conditioning, and more energy. There is everything to gain and nothing to lose by participating.