In those situations you acquire the 'right' to the stock over time (this is called vesting). And when you vest stock the IRS treats it like income and it gets added to your W2 as such.

The 83(b) election allows you to take the entire tax hit immediately even though you don't have the ownership rights on the stock yet. You need to come up with the tax payment but since you "own" the stock even when it vests you won't pay additional taxes, and your ownership starts the clock on long term gains (vs short term gains).

If the stock is going up an 83b can save you some money, if it is going down it makes it more complicated (you can write off up to $3,000 of "loss" per year of stock which is worth less than the 83b election price. I got to do that for just over 10 years on my dot com era 83b stock election.

Generally places like Facebook or Google will sell some of your RSUs as they vest to cover the tax hit so its pretty invisible to you.

Edit: I should add that this only matters for AMT purposes. If you early-exercise ISOs then the 83(b) election doesn't change anything for regular income tax, but does effectively accelerate your AMT income.

Restricted stock, notably, is not the same as an RSU (Google's "GSU"); one of the reasons an RSU exists is to simplify taxes, and because you are not issued stock at the time of grant there is no 83(b) election for RSUs.

Think of restricted stock as a chunk of equity set aside for you that you gradually vest rights upon. RSUs, on the other hand, are a promise to give you equity on a similar vesting schedule but the equity is not set aside. The effect on the valuation of the security with regard to your taxes in both scenarios should be self-evident.

Source: I've done this. I've early exercised ISOs and filed an 83(b).

It is my understanding that you don't file an 83(b) for ISOs or any other type of options, only for actual restricted stock.