Hillary set up the private server to avoid oversight, and conceal the special deals she was making to those who donated to the Clinton Foundation, also the same reason she didn't have an inspector general her entire 2009-2013 period she was at the State Department. The only period the State Department didn't have an inspector general.
Howard J. Krongard, who served as the agency’s inspector general from 2005 to 2008. “It’s clear she did not want to be subject to internal investigations,” Krongard said.
While Hillary's intent was to avoid oversight there is evidence Huma Abedin intentionally copied the secret documents so they wouldn't require decoding and then sent them over Hillary's private server. There is in fact at least one email discovered where Hillary directed an aide to do exactly that. While Hillary did it to carry out her corruption, Huma Mahmood Abedin who is a devote Muslim with close ties to Iran and Muslim Brotherhood supporter may have done it to feed secrets to Iran.