The money that goes towards the health insurance isn't subject to income tax, so it's cheaper than taking the money and buying yourself.

employer provided health insurance:

    $50k salary
    $5k health insurance
    $50k taxable income

buy your own health insurance:

    $50k salary
    $5k in lieu of health insurance
    $55k taxable income

But it's better/worse than that. Because they won't pay you $55k if you decline healthcare. Instead, you get paid $50k with the option to get healthcare (take it or leave it).

My family's insurance healthcare isn't tax-free, which would mean saving $2k or $3k or whatever. It's over $10k of insurance a year that I pay less than $2k for.