The number one reason for buying a new generator is the fact that it comes with a standard manufacturer's warranty. The length and terms of each warranty varies greatly but is typically between one and two years, depending on the intended usage of the generator. And these warranties do not come with a substantial increase in cost.
While they do not usually come with a warranty, it is often extremely advantageous to buy used electric power generators for a number of reasons. A key one is of course that used generators with low hours on the engines are often available at very attractive prices.