Where there is no other electricity source or when there is not enough amperage, a generator provides what is needed. They can be a primary source for businesses, homes, fairs, or movie sets but are often used when the power goes out in a natural disaster. Mounted on a trailer, one of these can power a commercial site that is small to medium sized.
Portable generators can power things such as most standard 120/240v electric power tools, lighting equipment, fans, blowers. Some of them have automatic idle control as fuel conservation and noise control features. They are typically small enough to be carried by one or two adults.