It's about accountability but also marking progress. Sitting here doing my exercises every day, I couldn't tell how much real progress I was making. It helps to have someone to mark your progress and give you the next steps.
In terms of why we go to rehab/PT I'm sure it has a lot to do with our culture...we American's and our Type A/overachiever vs. slacker/couch potato mentalities. We tend to see things as all or nothing (active or lazy, motivated or not), and our doctors also tend to be unnaturally attached to the outcome and progress of "their" knees...Egos, you know. Not saying I like it or agree with it, just saying how it is (somewhat exhausting to tell you the truth...)