Just to point out why this is significant, the Russians didn't just access voter registration databases, they had the ability to alter them. Once they had the data, it would show who voted and in what neighborhoods, they would know their age and other key details. They could then delete names, or change addresses, or make other changes that would get people turned away at the polls. Some people might wait around to file a provisional ballot, and others would leave in frustration.
Do this to enough people spread out over multiple precincts and you change the vote totals. That doesn't even include hacking the actual voting machines, which has been demonstrated before by university researchers.
Votes were stolen.