It reduces anxiety. Say you are slightly unhappy at your current job. On a normal stock plan, you might start feeling that "golden handcuffs are tying me to this company", which makes you feel even worse about your position, and might actually precipitate you leaving.

Now, consider this policy instead. You'd get a feeling of "I could leave if I really wanted to, nothing is tying me to this place". You get a greater feeling of agency, of personal freedom. That relaxes you, and you feel more comfortable staying with the company.