If you accept payment from an insurance company and sign a release, you are giving up your right to make any further claims based on the incident they are paying you for. That means that if you sign a release and later discover that your injuries are more serious than you thought, you won’t be able to make another claim or sue for more damages.
You have to remember that the insurance company is not on your side. Their job is to take in as much money as possible and payout as little as possible, just like any other business. It is in your best interest to consult with a qualified bicycle accident lawyer before signing any paperwork sent to you by an insurance company.