Rachel Louise Ensign & Melissa Korn:

So Harvard has put you–or someone you know–on its waitlist. Great news! Or maybe not.
A spot on a waitlist from an elite school doesn’t necessarily mean a candidate is closer to the finish line. Some may be waitlisted, for example, because though their grades weren’t quite good enough or they didn’t take enough advanced placement classes, they still piqued the interest of admissions officers. Others are offered spots purely out of courtesy, such as family members of alumni or children of donors who failed to make the academic cut.
Schools often pad their waitlists to protect their “yield,” or the proportion of accepted students who choose to attend. They can admit fewer students on the first pass, to maintain their aura of exclusivity, then move on to the waitlist if accepted students turn them down.