Heat cables help, but they don't fix the root cause. They melt channels along the eave so water can drain instead of pooling behind an ice ridge — useful, but they don't stop the heat loss that creates the dam in the first place.
Where heat cables earn their keep is on the spots insulation alone can't solve: valleys, roof-to-wall transitions, and around dormers. We usually recommend them as one part of a fix — heat cables on the problem areas, plus attic insulation to R-49 and balanced ventilation to keep the rest of the roof deck cold. Cables alone on an under-insulated roof just run up your electric bill.