What is the part of "calculates the value of all benefits to be paid out in the future, due to past and future service" that you don't understand? Congress required all possible benefits present and future be calculated (subtracting only future accruals, i.e., gains in investments) and then that money had to be raised over 10 years. That is never required from companies, which have to fund only the portion of pensions that they effectively supposed to pay based on current liabilities, not what they will be projected to owe in the future.
You can find the explanation in several articles, for example:
"[the law] requires the self-supporting U.S. Postal Service, which receives not one dime in taxpayer subsidies, to fully fund its retirees’ health benefits for 75 years into the future. It also requires that money be set aside over a 10-year period, at a rate of more than $5 billion per year.
That means the postal service is now paying for the future health care of retirees it has not yet hired, and who in some cases have not yet been born. No other public or private company in the nation bears any kind of financial burden like that.
Even worse, none of that money is truly being set aside. Instead, it is going directly into the U.S. government’s general fund, and it’s being spent on current government operations. The set-aside is a theoretical accounting gimmick. Those future retirement liabilities are actually being added to the national debt."
https://roanoke.com/news/local/casey-the-most-insane-law-by-...