If you want to prove, when you retire, that part of your IRA withdrawal is non-taxable because you made non-deductible contributions, then you need your form 8606s. Without them, your entire IRA withdrawal is taxable, even though you already paid tax on some of the contributions, because the IRS does not keep track for you.
Likewise, if you want to do a Roth IRA conversion, you need the form 8606 to prove that part of the conversion is non-taxable. Without them, you will pay tax on the entire converted amount, even though you already paid tax on some of the contributions, because the IRS does not keep track for you.
