First, I'm sorry. I hope you are getting the treatment you need.
Second, check your policy. There are two reasons they might want the report:
1. Most likely, they are going to go after the other insurance company to recover the costs of your treatment from them. Even my health insurer recently sent me a questionnaire about DS's broken foot, wanting to know if it was caused by an accident and if so who was at fault (ummm, mother nature? A/k/a ice/mud on a playground? Yes, let's sue the school for daring to let the kids play. But that's another rant).
2. There might be limits on your policy that mean that it doesn't cover disability caused by other people. In that case, the insurer might deny coverage, and you would have to make a claim on the other driver's insurance company to recover the costs of your treatment from them.
It is probably the first one; I would hope the insurance companies would fight it out amongst themselves and you wouldn't need to be directly involved. That's the way it should work, at least. But in any event, you need to provide the police report, because I guarantee you that your policy requires you to cooperate with them on any claims and provide any information they request, so if you don't send the report when they ask for it, they could use that refusal as an excuse to deny your claim.