Narrow depth of field
One reason some professional lenses are so expensive is that the diameter of the lens is enormous, making the glass more costly. So why do they do it? The larger diameter lets more light into the lens, resulting in better low light performance and narrower depth of field, assuming the lens setting (f-stop) is wide open.
What is narrow depth of field? Contrary to what most of us expect regarding focus (everything is in focus), the narrow depth of field shot puts most everything out of focus except the subject. This is a good choice when the background is full of stuff you would like to eliminate.
Geometry lesson: Draw a line out of the lens to the subject and label the distance x. Now draw a surface perpendicular to that line. That surface is called the plane of focus. The depth of field is some delta of x where the focus is judged acceptable. With some lenses this could be plus or minus one inch from the plane of focus.
As confusing as all of this can be, one look at the photograph on the left should convince you that narrow depth of field can have a dramatic impact. See how the focus gradually gets blurry as the distance from the face moves towards the background. Especially note how the blurry background seems smooth and creamy. That smooth characteristic is called bokeh, another prized feature of an expensive professional lens.
Taken with a Nikon 70-200 zoom @ 200mm f 2.8 1/45 sec, hand held with available light crawling on the floor with baby.