Stock grants let them push part of your compensation for work done today out to next year, or beyond. The money (stock) that's coming in the future keeps you at the company.
At least at Google, most people viewed it as a part of your compensation. Similar to a raise. It was done every year, and you'd eventually have several overlapping GSUs at once. It was almost more a reward for tenure than anything else.
The thing that kept people around was the annual bonus. It was always interesting to see who quit the day after the bonus was paid.
At least, that's why I thought it was done.