So it's interesting, for both of these the explosion in their popularity actually happened in WW2, although both existed before WW2.

During WW2 there was a labour shortage as the wartime economy was booming. President Roosevelt signed into law the Stabilization Act of 1942, which froze direct compensation and bonuses, but exempt to the freeze were benefits like health insurance and pensions. So that's what employers started using to attract employees.

https://www.chicagotribune.com/opinion/commentary/ct-obamaca...