Good question! Your boss likely asked you directly because the easiest way for employers to verify someone’s car insurance is to ask them for proof of insurance. The only other way for employers to gather employee insurance information would be through the police, the DMV, or the state department—all of which have the right to deny requests for information.
Plus, the biggest reason your employer cares about your insurance status is their liability. If you get in an accident while driving for your company, then your company may be liable for your actions. To mitigate this, your employer may be able to add you to their commercial auto insurance plan or business liability policy; it might be worth asking them.
Hope this settles your curiosity!