For anyone already paying attention, this was not a surprising revelation. The whole ongoing scandal had all the signs of classic Putin tradecraft but his fingerprints were lacking. No more. The only real surprise was that he wasn't able to completely cover his tracks.
This leaves us with the question of why. Part of the reason is because Putin is well documented for loathing and fearing Hillary Clinton. If he could tilt the scale to Donald Trump, he could settle an old score with Clinton for what he perceived as slights.
He also clearly thought Trump in the White House would be better for Russia. Trump has always spoke of the Russian near-dictator with fond and almost loving admiration, refusing to criticize him in any manner publicly.
This suggests Putin thought Trump would be weaker than Clinton and much more easily manipulated. The first disastrous five months of the Trump presidency have proven him correct, if that was his goal.
And last, but not by any means least, Putin saw it as a way to destabilize American democracy for the benefit of Russia. This is classic Putin style and he's done it before with other countries. The weaker the US becomes, the better for Putin.
Now Trumpers will tell you nothing is wrong with Russia, that it deserves to be our close ally. That, of course, is rubbish.
Putin is almost but not quite a dictator. Give him time. He openly persecutes LGBT citizens and others. Russia is not a free country. Critics are silenced or murdered. It's a kleptocracy and a thugocracy, not a democracy.
The way the Russian government is run is an anathema to the American government. Every single American, no matter your political beliefs, should be outraged that a near dictatorship wants to weaken this country.