Final answer:
The rider allowing policyowners to buy extra insurance without proving insurability is the guaranteed insurability rider. It is advantageous for policyholders who want to increase their coverage in the future without health re-evaluations. This contrasts with coinsurance where costs are split between the insurer and the insured.
Step-by-step explanation:
The rider that allows the policyowner to buy additional insurance at certain times without proof of insurability is called guaranteed insurability rider. This type of rider is particularly beneficial because it enables individuals to increase their coverage as their needs grow, without having to undergo a medical examination or provide any health information that could potentially affect their insurability. For instance, a person with life insurance might opt for guaranteed insurability to ensure they can purchase additional coverage even if they have developed a medical condition that would normally make them uninsurable or subject to higher premiums. Coinsurance is a different concept where the policyholder and the insurance company share the costs of a claim. The knowledge that a person has about their own health or driving habits, which may not be fully accessible to the insurance company, is a reason why riders like guaranteed insurability are valuable.