I assume you're referring to the 1-year deals that some places offer. These days they're mostly for people who didn't match into a program and are looking for a worthwhile way to spend the year while waiting to re-apply. If you do an internship at a program you're interested in, it's a chance to "prove yourself" and show them you're a hard worker. You can also get good recommendations (hopefully) from faculty that know your work better than your dental school faculty. I think most program directors would tell you that an oral surgery internship is much more useful than a GPR, even though we have some GPR guys who matched in my program.
Our program has at least one per year. I don't really think they're all that hard to get into. Basically, the day after match program directors start looking for the one year interns. The directors are looking for that guy as hard as the guys that didn't match are looking for something to do.