Sometimes it is easy to grab a camera and just drive or walk around until something catches the eye. No plans, no preconceived ideas, just whatever one finds of interest along the way is what one photographs. There is nothing wrong with this, and, in fact, I enjoy the adventure of it.
|Two Ostrich - Tehachapi, California|
I think there are a few reasons for this. First, one has time to pre-plan and pre-visualize. I cannot say enough how important that is. Second, because you have a plan and goal, it's easier to avoid distractions. Finally, when you say that you are going to photograph a particular subject at a certain place and time, you are giving yourself limitations, which will improve the art.
Now I'm not suggesting that one should only do planned shoots and put the camera away at other times. But if you are not from time-to-time giving yourself the limitation of planned shoots or outings, I believe you are missing out on the blessing of the limitation.