Picture this: a magnificent red rose, strikingly bright, exquisitely layered, every detail in sharp focus. Now picture a distracting mess of grass, weeds and other flowers directly behind the rose. Your eye can't properly focus on the rose thanks to the background. That's a problem. The solution: knowing how to properly frame the subject with depth of field. Shooting with a shallow depth of field will keep the subject in focus while blurring out the background.
Shooting with a large aperture setting creates a shallow depth of field that isolates the subject. The quality of the lens aperture can also impact the shot. The aesthetic quality of blur, aka "bokeh," is affected by how many blades there are in a lens aperture. Pleasing blur keeps the background from interfering with the intended subject.
That said, don't focus every shot on a single subject with shallow depth of field. Simply shooting from the proper angle can minimize background distractions, and sometimes a photograph will turn out to be more interesting with multiple plants in the picture. For those shots, use a medium deep depth of field to keep more of the frame in focus.