It is not about needing to know. In most states LE has access to insurance information, and the production of an insurance document is redundant. However, as an example, in New York, failing to produce the insurance document (card) is punishable with up to 15 days in jail, a fine of up to $1500 and a civil penalty of $750. Production of the insurance document is INDEPENDENT of whether the car is actually insured or not.
For example, not producing the insurance card is presumptive evidence of failure to have insurance. However, producing the card, even if the insurance were to not be in effect, has the affirmative defense of not knowing about the fact that the insurance is not in effect.
Also note, that the driver at the roadside stop, is required to produce the card evidencing insurance, but it is the owner who is required to have insurance in effect.
It is also worth noting that most drivers, covered under a policy, will have coverage for a "borrowed" vehicle.
When you are detained (seized even) in a traffic stop, most likely the LEO is "regulating traffic" and can request these documents by asking you. You are presumed to be operating in traffic, and therefore your compliance is required.
Current registration is also in a database which is often available in the patrol car. Many cars have their database updated daily, and if one's registration was revoked or suspended that would be available to the LEO within 24 hours. However in most states the requirement to produce a registration document still exists, and if the LEO so chooses they can cite you for failing to produce the document, even though they know that the registration is valid.
So in short, a traffic stop is done under the presumption that you are engaging in a regulated activity, which is called "traffic" or "motor vehicle traffic" and that you are subject to the regulations which apply to that regulated activity. That includes production of insurance, registration and driver license documents, and could include other documents. Failure to provide those documents makes one subject to consequences under the regulations, statutes, codes and perhaps even policy.