The interprovincial insurance card is a piece of paper that comes with a renewal of a Canadian auto insurance policy and is required to be in the vehicle as proof that the car is insured. Anyone driving in Canada needs that sort of proof but this would only be in an issue in an accident or if there was some police question about ownership. Every US insurance company has access to giving these to its policyholders at no charge - it might be even be useful while you drive in the US as proof of insurance. It is necessary to have a passport (or equivalent) to re-enter the US. So whatever you are bringing to get into Canada is not different. Effectively, Canada wants you to be able to get home so the border agents like to check before admitting you.