Why does TurboTax want to know the total value of all my Traditional IRA accounts on 12/31/2021 when I converted a small part of the Traditional IRA balance to Roth?

TurboTax needs to know the value of all your IRA accounts at end year when you convert part of your IRA to a Roth IRA. This is in order to calculate the basis of the converted part by applying the pro rata rule.

When your Traditional IRA has basis (from the nondeductible contributions) as reported on form 8606, you have to apply a prorated amount of basis to the converted amount.

Please read this excellent Community answer by Champ dmertz which explains the pro rata rule.