One of the basics in photography is determining what you want to focus on/what the subject of your image will be.  At the same time, one of the real fun aspects of photography is knowing how those items that are not in focus can enhance the image.  Here are two examples:

columbine

This first image is a columbine.  The flower (and pod) are sharply in focus and the background is mostly not.  The lack of a distinct background helps the foreground flower pop.  The out of focus flowers in the backgrounds serve as echos of the foreground in shape and color.  If they were sharper, however, they would draw the main focus (the foreground flower) back in the image and muddy the impact.

This second image (roses) shows the foreground out of focus and the sharp background sharp.  Here the indistinct blocks of color caused by having foreground roses extremely out of focus serves to frame the focal roses and provides a nice color difference that (I hope) complements and enhances the background.