What can cause Stuttering?
Until very recently, there have been no underlying reasons known that caused people to stutter. However, it was very clear what made stuttering better or worse. Emotional factors are one of them. If you make someone stressed or nervous, his or her fluency is going to go down. For example, putting a person who stutters in front of a group of people, talking on the phone, or any new situation will decrease fluency. If you distract a stutterer or speak in unison with them their fluency could go up.
"In 2010, for the first time, NIDCD researchers isolated three genes that cause stuttering."  In doing so, they have learned that about 50–70 percent of the reason people stutter is due to genes. The National Institutes of Health, the world's largest medical research center, has spent over 13 years looking for the genes that cause the disorder.  This finding could lead to further study and options for treatment. For more information on this new discover visit the National Institutes of Health.
Research is important because we still don't know that much about therapy. Why therapy works well for some people, moderately for other, and still for a few, therapy doesn't work at all. Research can help answer this mystery. Efficacy studies on therapy will also help figure out if different genetic mutations respond better to certain types of therapy. Research is also important for developing drugs for this disorder. We are still a ways off from finding a drug, but this genetic link will bring us closer than we've ever been. 
Stuttering as a Disability
According to Barry Guitar in Stuttering: An Integrated Approach to Its Nature and Treatment, the disability of stuttering is the limitation it puts on the individual's ability to communicate. The limitation is both due to the severity of stuttering and how people who stutter feel about themselves and how others react to them. Stuttering can often result in a disability, but the magnitude of it can be reduced, even if the person still stutters.  In the most severe cases stuttering may even result in handicapping conditions because it can limit how well a person can communicate at home, school, on the job, on the phone, or just out in public.
 "Stuttering-An Overview by NIH."
 Drayna, Ph.D., "Telephone Interview with Dr. Drayna, Chief, Section on Systems Biology of Communication Disorders at the National Institutes of Health."
 Guitar, Stuttering: An Integrated Approach to Its Nature and Treatment, 18.