In most cases, you are granted a notional dollar amount that is immediately turned into a concrete and fixed number of shares that then vest over the next 4 years.

Then, any share price appreciation on the shares is captured by you at vesting, rather than being paid in cash (the value of which has been inflated away) and then purchasing shares/index that has risen in the last 1-4 years.

If you are paid in cash, you will be buying fewer shares per dollar (and per year) rather than getting the same number.