People commit robberies for various reasons, but mainly to gain something of value. Some criminals may commit robbery because they are interested in violence, rather than financial gain.
When criminals commit crimes, including robbery, they do so for specific reasons. For instance, a drug addict may commit the crime in order to support his or her addiction. Addicts may steal money or something of value in order to purchase drugs when they have no other means of obtaining them. The majority of people who commit robbery only do so once. It is common, however, for those people to have committed other types of lesser crimes, including stealing.