The conventional assumption is that Julian Assange is working for the Trump campaign. Likewise, those who subscribe to the idea of Russian hack sources are thinking the same direction, although if true, Russian motives are sure to be more complex.
Julian Assange has already declared a reason that he is releasing emails damaging to Hillary; it is to paraphrase: her globalist military ambitions, or her track record and willingness to assert her will for causes not directly associated with self defense.
Ergo, the reason we should be thankful for the email leaks sponsored by Assange is that he is preparing for an America run by the despotic Hillary by showing her corruption up front. I doubt that Assange has ever felt confident in a Trump win or that he would prefer tyrant Trump over tyrant Hillary. He is a realist, he sees a Hillary win as likely, perhaps based on extensive knowledge of what the globalist are doing in the back room, and therefore is simply preparing America and the world for what comes next. Thank you Julian.
Postscript
See also this editorial by Peggy Noonan