Re: PhD
^ among other things... but often times, the incoming students look for supervisors who have "similar" research interests and once they come in, they're apprised of the research projects that are underway, and typically, the supervisor has some research funding to pay the grad students which is why they're asked to consider a dissertation area that has a close connection to these projects.
I'm not saying it happens in most cases, but it does happen - especially if you're going into your Ph.D. without knowing the personality and preferences of your supervisor who may or may not be flexible enough to grant you your particular choice of research area. Remember that grad students are paid in part through the grants of their supervisors and they command some power over what you do while you're with them.