How does this make sense, though?

> "The war economy is an entirely different ballgame," Thomasson says. The government rationed goods even as factories ramped up production and needed to attract workers. Factory owners needed a way to lure employees. She explains that the owners turned to fringe benefits, offering more and more generous health plans.

Why is a healthy plan better for employers than paying cash and letting employees buy their own health care / insurance?

The 1943 IRS ruling that health insurance is tax free, and the legislature failing to correct that, is more of a problem. This blatant failure isn't an "accident", it's bad lawmaking that biases toward business, against people.

Wage-price controls prohibited that.

and letting employees buy their own health care / insurance?

Risk-pooling, information asymmetries, "shoe-leather" frictions (having to shop around, individually, for everyone), bargaining power.

Healthcare markets poorly.