1) It subtly anchors execs and HR to accept your perk as reasonable, especially because the people paying attention to Glassdoor are far removed from your manager and don't actually know if you have those perks or not. "Since our company already does it, it must be a good idea," thinks the exec.
2) Your coworkers see the reviews and assume the perks are true but unevenly distributed, thereby further ingraining the perks in the company zeitgeist.
3) If you are eventually found out, nobody will ask you to write reviews any more.
and 4) Candidates, including good candidates, will come to interviews and say things like "I've heard you have 20% time, that's a great idea, tell me more about it." This fans the flames in favor of your perks.
I used this technique as part of a wildly successful give-us-more-perks agitation campaign in my previous job.