Amateurs do microscopy because they enjoy it, professionals to earn a living.
Although this simple statement is true, it leaves a great deal unsaid. Most professionals enjoy microscopy every bit as much as amateurs, though some of the sample preparation and statistical analysis may be repetitive and tedious. Professionals are necessary because there's important work to be done by trained staff using equipment that is often expensive and difficult to use correctly. As in most walks of life, professionals can devote all their time to a specialized area so that they become highly skilled and effective in their work. Modern society depends on microscopy in many ways.