So they can take you to small claims/civil court if you or your insurance doesn't pay up.

And it helps in collection efforts in general. There is no benefit to you in divulging it, only potential harm. So don’t do it.

I think some just want a stable unique identifier to prevent duplicate accounts. Many use Name + Date of Birth, but that's not always unique.

Often this is how they verify you are insured.

For medical insurance it’s usually only needed if they you don’t have your member ID number or just recently enrolled. More common for vision or dental insurance to use SSN though.

They have no problem verifying my insurance without a SS #.

Depends on the insurer, probably. That's how my dental is verified since my dental insurance doesn't bother sending out a card...