On the eve of the Democratic National Convention, a series of e-mails from the Democratic National Committee was publicly released. They showed no wrong-doing by the committee, but did contain some stupidly embarrassing e-mails from some committee members (especially one from the chief financial officer). The release was obviously designed to hurt the candidacy of the Democratic nominee, Hillary Clinton.
We now know those e-mails were discovered when Russians hacked into the DNC computer. The question that brings up is -- why would the Russians want to hurt Clinton? Why would they want to interfere in the United States election? The obvious answer is they were trying to boost the candidacy of Donald Trump.
Some people posed that they wanted to help Trump because Trump has significant holdings in Russia, or that Russian investors have significant money invested with Trump in this country. The Trump campaign was quick to deny that. Personally, I think that was a straw man argument (probably designed by Trump supporters). If Trump does have holdings in Russia or Russian investors, I doubt making him president would help that (because he would need to divorce himself from his business interests if elected).
But that doesn't mean Russia doesn't have an interest in seeing Trump elected president. For one thing, Trump has expressed his admiration for Putin. It would not be wrong for Putin to think he could get along with Trump (and use him) better than he could with Hillary Clinton (who would continue the foreign policy of Barack Obama).
But perhaps the most important reason is Trump's attacks on NATO. Trump has said NATO is outdated, and even suggested that he would not act to defend some NATO countries (who he claims are not paying their share of the burden). That had to be music to Putin's ears.
NATO is the primary defense of Europe against Russian aggression, and Europe depends on the United States keeping its word and honoring the treaty. Hillary Clinton would do that. Donald Trump has said he might not. Donald Trump would make NATO weaker, and maybe useless.
That's the real reason Russia would act to help Trump -- because Trump admires Putin and does not believe in NATO. Trump would put Europe at the mercy of Russia.