There are so many rules on composition that are supposed to be followed (and broken from time to time) that it can be hard to keep track of what you are supposed to be doing and why you should or should not follow that rule.
One common rule of advice is to fill the frame. Depending on what genre you are shooting, this can mean different things. In many genres, you can almost always make your shot more interesting by moving in closer, but that is not always the case. I like to think of filling the frame by eliminating all distractions from your image.
Whether you crop before or after you actually take the image, the end result is your decision on how you want to fill the frame. Since photography is a form of storytelling, how you frame your photos determines what story you are telling.
While hiking in the Valley of Fire State Park north of Las Vegas, I came across this little cactus. I set up the Oben tripod I was reviewing and looked at different angles and focal lengths to decide what story I was trying to tell. I ended up with three images that all tell a different story. The above image shows the entire cactus with its surroundings. It tells the story of a thriving and healthy cactus growing out of a rock. it emphasizes life in the desert.
The Second image in the series is zoomed in to show just the cactus without any distractions. The cactus becomes the story. There is no way to determine where it is located at or what it is surrounded by. I think the color of the cactus is the star of the shot in this image
The final image is not too different from the second, I moved my camera just a few inches closer, but the story has changed. You can no longer see the shape of the cactus. You still have the red color, but is not as prominant as there is more white and green filling the frame. The story here is all about the needles.
With almost any image we make there are different stories to tell. It is all about how we fill our frame that tells the story so make sure to be conscientious of your entire frame to determine if it tells the story you want.