The meeting helps establish a few critical facts. The first is simply that contacts between Russians and campaign officials did take place. If you are seeking to prove a criminal partnership, evidence that the alleged partners had private meetings establishes the opportunity to reach an agreement.
Of course, there already was evidence of other meetings between Russian officials and members of the Trump campaign. The crucial new detail about this meeting is that campaign members now admit it took place after they were told that Veselnitskaya was offering compromising information about Clinton.
This fact is significant regardless of what happened at the meeting. Proving a defendant’s state of mind is key in any criminal case. This meeting provides critical evidence about the state of mind of Trump representatives: They were willing to hear what a Russian individual had to offer about their opponent.