An SR-22 certificate of financial responsibility is generally referred to as insurance, but it is actually a special document that is purchased from insurance companies and does not provide any insurance coverage at all. The form is a consequence of a serious traffic offense, such as a DUI or reckless driving charge. It can also be sentenced against you if you are convicted of driving without proof of insurance. If you have been instructed to purchases an SR-22, you have to keep it for the duration of the sentence, in addition to your regular car insurance.
You can be required to have SR-22 coverage even if you do not currently own a car. This usually happens in situations where multiple convictions for the same charge have already been leveled and there is no confidence in your ability to refrain from driving a vehicle. In this case, the judge or DMV can order you to carry SR-22 coverage to make sure that you will have the required insurance even if you are driving illegally.
When you have been ordered to carry an SR-22, it is purchased through the insurance company, and the company notifies the DMV that the certificate has been issued. If you allow the SR-22 to lapse, the insurance is required by law to report it to the DMV and your license will usually be suspended or revoked.
Remember, you will still need to carry at least the required state minimum insurance in addition to the SR-22. The SR-22 simply stats that you are in compliance with the law; it does not eliminate the need to buy the coverage required. The bottom line is that being required to carry an SR-22 will not do anything except keep you in compliance with the law, and even that requires you to have any other car insurance coverage necessary in your state.