Why should health insurance implement the proportional payment clause?

In health insurance, because people's health is the target, there is no question of whether to insure in full; At the same time, because the danger of health insurance is not easy to control, in most health insurance, there are clauses that the insurer pays the medical insurance premium in proportion. The proportional payment clause is not only conducive to protecting the interests of the insured, but also conducive to the insurer's control of medical expenses.