Well that is the Number One Reason why they are going immuntheraphy the side effects are minor compared to Chemo and Radiation. This is your own body fighting the cancer.
I don't think "Minor" is accurate. A good friend just went through this to beat a strange form of metastatic blood cancer, and he was in the ICU for a week as a direct result. (and essentially comatose) It was worth it, since he went from, "Plan your death" to, "Full remission!". Still, he was strong going in, a lot of people would have died.
Right now, there's still a lot of art to the balance between having your immune system fight hard enough to kill the cancer, without setting off a cytokine storm that kills you. There's also the issue of longer term autoimmune complications, which I suspect will turn out to be the "secondary cancers" of the immunotherapy world.
There's nothing to say that leveraging your own immune system will necessarily reduce the frequency or severity of side effects relative to current therapies. Your immune system is quite capable of killing just about any cell in your body, foreign or not. The multitude of different autoimmune diseases are clear evidence of that fact.
