Forms 5498 report to you and to the IRS how much you have contributed to your Roth IRAs. If you make a nonqualified distribution, the IRS generally relies on you to accurately prepare Form 8606 Part III where you must indicate the amount of your basis in Roth IRA contributions and conversions, but the Forms 5498 give the IRS the necessary details to determine if your entries on Form 8606 are accurate, should they choose to check.
