Nitpick: Non-public companies don't give RSUs. They either give stock options or "restricted stock", which is actually a completely different thing from RSUs. Either way, the basic intent is to ensure that, for tax purposes, the grant has zero value, because if they gave you a non-zero-valued grant then you'd have to pay taxes on it and if you can't sell the stock to cover taxes that is going to suck. Once public, the company can give RSUs because they are able to actually take tax withholding out of them (by selling a bunch).

I would argue that RSUs are rarely something the employee wants, compared to equivalent cash. At a big public company, you aren't likely to be able to affect the stock price with your personal performance, so it's not an incentive. You also likely can't predict whether the stock is likely to go up or down -- if you can, you're in the wrong job.

I would speculate that the appeal of RSUs to Google is as a hedge -- if Google's stock price goes way down, at least they'll get a break in employee compensation. Otherwise I honestly don't know what the point is. Just give people cash.

Startup equity is a completely different beast. You're getting a grant that is (in theory) worth $0, but you may actually be in a position to predict whether that startup is going to succeed, and even to influence the outcome. Maybe. That said, if the company doesn't tell you what percentage of the company you're getting, then it's a scam and you should treat it as $0.

(I'm a former Google employee and current startup founder.)