It’s basically that you’re forced to choose between a non deductible Trad IRA, or a Roth conversion of a non deductible Trad IRA contribution, which assumes you have no other Trad IRA funds.

Same for MBDR: once you’ve maxed the pretax 401k space the only thing left (if your plan even offers) is after tax. And since after tax earnings are pretax, you may as well convert to Roth to make the earnings (eventually) tax free.