A Roth IRA contribution does not actually go on a tax return, but you can enter it anyway to:

1) Tell you if your income qualifies you for a contribution and warn you if it does not.
2) Check if your income exceeds the limit to contribute to a Roth.
3) Track your contribution year-to-year if you use TurboTax every year.
4) Add the Retirement Savers Credit if you qualify.

If you entered it than you might have a  "IRA Contribution Worksheet".

2018 MAGI

2018 adjusted gross income before any IRA deduction plus all of the following

foreign earned income exclusion

foreign housing exclusion or deduction

excluded employer-provided adoption benefits shown on form 8839

excluded US savings bond interest shown on form 815

student loan interest deduction

tuition and fees deduction 

limit $5500 ($6500 if over 50 at year end) or compensation if lower than those other limits

phase-out if income too high

1) magi

2) $120,000 if filing single or head of household,  $0 if if married filing separate, $189,000 if married filing joint

3) line 1 less line 2

4) $15,000 single, head of household, or married filing separate and lived apart from spouse all of 2018, $10,009 all others

5) divide line 3 by line 4 round to 3  places do not enter more than 1.000 

6) lesser of taxable compensation or $5,500 ($6,500 if 50 or older at year end)

 7) multiply line 5 by line 6 

😎 subtract line 7 from line 6. round up to nearest $10. if less than $200 enter $200

9) enter the contributions for 2018 to other ira's do not include simple or sep contributions

10) subtract line 9 from line 6 

11) reduced Roth lesser of line 8 or line 10