Most people want to deduct Traditional IRA contribution to lower their tax at the time of the contribution, but when you withdraw the money, you must pay the tax that was deferred.
You can, elect not to deduct the contribution, then it becomes after-tax "basis" in the IRA that will not be taxed again when withdrawn. It is not common to make such non-deductible contribution, since Roth IRA's are after-tax and have many advantages over Traditional IRA's if a non-deductible after-tax IRA is wanted.
Some people whose income levels prevent them from directly contributing to a Roth, will make non-deductible after-tax Traditional IRA contributions and then immediately convert that to a Roth as a "back-door" method of making a Roth contribution.
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