Good question. Let me start by saying that for a person with enough money saved up to live off of for the rest of their life in a mixture of taxable (post-tax), Roth, and 457 accounts, they have already won the game. There are essentially no bad options, just different degrees of optimal.
So why would I spend 457 money first? Because money in my 457 account, like all traditional retirement accounts, is money that is eventually going to be subject to income tax no matter what I do. Since we have a very progressive income tax structure in the USA, the way I can minimize the total amount of tax I'll pay on my 457 balance is to spread my withdrawals out over the maximum number of years so most of it is in low income tax brackets. I'd pay much less total income tax on $40k of income a year for the next 40 years than I will on 39 years of zero income and one year where I take out and am taxed on $1.6M. Since the year I'll die is probably outside of my control, the only way I can spread my withdrawals over more years is to start tapping into the 457 earlier.
Compare that to my Roth funds, which neither I nor my -- entirely hypothetical unfortunately -- heirs will ever have to pay taxes on.* I can maximize the size of that tax free benefit by waiting as long as possible to tap into those accounts but realistically I can spend that money whenever I need to without it messing up my income planning for the year. Until I choose to spend it, it provides flexibility in the face of an uncertain future.
Post-tax (taxable) is somewhere in between. Unlike a 457 (or traditional IRA/401k) it isn't subject to income tax. If I suddenly needed $300,000 next year (maybe I'm buying a house with cash? Or a golden visa?), withdrawing it from a 457 would mean paying $75,000 in federal income tax. Sell $300,000 in appreciated stock in a taxable account would mean I'd likely end up only owing on the order of $16,500 in capital gains taxes (Assuming the stock had appreciated 50% since I bought it). In addition, while I'll eventually have to pay capital gains tax on my taxable investments if I sell them in my lifetime, if I delay selling stock so long I die before getting around to it, my hypothetical heirs don't have to pay any tax on them at all since they'll benefit from stepped up cost basis.
So there you have it. Traditional IRA, 401k/457 accounts WILL be subject to a progressing income tax eventually so there is value in spreading the use of the money over as many years as possible. Appreciated stock in a taxable account MIGHT be taxed eventually (but if so at a low rate) and it might not at all, and Roth certainly won't be, so there is no reason to spend down either earlier, and potentially some modest benefit to delaying doing so if you have other pots of money, like a 457, that can cover your needs.
*So long as I don't withdraw more than my initial contributions before reaching traditional retirement age.