I also work for a largish company that has had a vacation buying policy for as long as I've worked there. Our policy allows the purchase of up to 1 week of vacation via paycheck deduction. We can cash in unused purchased time up to November and get that money returned in our December paychecks.
At my workplace, I see three main categories of people taking advantage of this policy:
- people who always like or need more vacation - least germane for selling the policy
- people buying the vacation "just in case" then cashing it in at
- newer employees
The key group, and the one I think worth focusing on, is newer employees. They have the least vacation, and they may have a lot to accomplish outside work all at once - house/car/DMV/bank/etc.
At the same time, new employees may be wary of rocking the boat. As Enderland notes, unpaid time off, at least where I work, involves discussions and planning and permissions. A lot of new employees may be reluctant to raise the issue while they're still learning how the corporate culture works. Vacation buying is just an option they can choose along with other flexible benefits - it sends a message that this is normal, other employees do this too.
For employees who start during the fourth quarter, this is also the vacation time that lets them spend year-end with family, rather than being unproductive in the office while all their mentors are on vacation.
I'm in group 2, I buy the week every year as cheap insurance and cash it in at year-end. The company gets an interest-free loan on 2% of my salary, and I get flexibility if I need the time. I did have a minor crisis one year, and I didn't have to explain why I needed more time. In turn, HR didn't have to OK a two-day unpaid leave that wasn't worth anyone's discussion time.
Purchased vacation is a cheap way for the company to offer flexibility and make its employees happier.