<p>Its not even so much a back door as much as getting information about the people you might work with for a long time. For one thing, if the one or two labs you really want to join don’t have space or funding for a new student, you might be wasting your time applying, although of course its up to you to decide. Its also good to see what current and future directions a lab is working on; a lab you weren’t interested in might be starting work that you love, or a lab that historically did stuff you love might be switching gears to something that doesn’t really interest you. At the very least, contacting professors will get your name into their head and will probably help get your application seriously considered.</p>