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.