Before Roth IRAs were invented, there was the situation of taxpayers being able to contribute to a traditional IRA, but exceeding the annual limit for deductible contributions. This was based on the MAGI of the taxpayer (and spouse) and whether the taxpayer or the spouse was covered by a retirement plan at work.
Had Roth IRAs been invented at the same time as traditional IRAs, then it wouldn't have made a lot of sense to also allow nondeductible contributions to a traditional IRA, which is the source of your confusion.
It is certainly much easier today to make all your deductible IRA contributions to a traditional IRA and the nondeductible ones to the Roth IRA - this would remove the need to using form 8606 to track the "basis" (amount of nondeductible contributions in the traditional IRA).
However, given that the traditional IRA predates the Roth IRA by more than 20 years, rules had to be invented to accommodate traditional IRAs that already had nondeductible contributions - hence form 8606.
NOTE: IRA contributions are not reported on your W-2, because it is a personal account not done through your employer. Instead the deduction to a traditional IRA is shown on line 19 of Schedule 1 (1040).
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