If you consider the case of publicly-traded employers (there's a different argument for pre-exit startups that comes out to a similar conclusion): they could give you your compensation entirely in cash, because shares are pretty liquid. And you could buy stock in your own employer. But very few people think that's a good idea, because now you have correlated risk across your own actual job (both continued employment and things like the ability of the company to give large bonuses/raises) and your investments. So the company effectively forces you to buy some of their stock and not sell it for some period of time. You know exactly how much you're forced to buy because, again, the company is publicly traded and you know what that's worth; you just get it in illiquid form.

So, by giving you part of your compensation in this funny form instead of in normal cash, they get you to feel internally conflicted about policies that improve your cash compensation (and other intangible benefits) at the expense of a theoretical drop in your funny-cash compensation. That doesn't benefit you (even if you were the sort of person who wanted to invest in your own employer, you're still free to do that on your own if you want, so forcing you to do it is at best equivalent to giving you the option), but it does massively benefit the people who own way more shares in the company than you ever will.