I got the shot for two reasons, I had chicken pox when I was 12 and my mother suffered terribly with shingles shortly before she died. YOU DO NOT WANT TO GET THAT DISEASE!
Techinically you and I already have the disease. Shingles is the secondary infection of Herpes zoster, with the primary infection being termed Chicken Pox. In the exact same way, cold sores are the secondary infections of Herpes simplex with the primary infection being termed foot and mouth disease. After primary infection the both viruses lay dormant in nerve cells where immune cells do not reach them. It is not known what causes the viruses to become active again.
Unfortunately I am a carrier of both. If I ever have children I will have them vaccinated for chicken pox directly. Now I wonder why there isn't a herpes simplex vaccine...