Whoever hacked the data was most likely part of an organization with political interest in the upcoming election. They have a stake in trying to discredit the DNC, and gain power for themselves. Of course it makes sense that Putin wouldn't want to hacker to be found, seeing as there is evidence that the Russian government was involved in the other cyber attacks on the DNC.
For authorities to track down who hacked the DNC data, they need to be aware of where in the world the perpetrators are located. If the hackers are working for another country, US officials need to know so that they can take proper precautions to protect sensitive data from spies.
It doesn't matter who hacked the DNC. This is because there were underhanded practices and secretive goings-on, by both DNC workers and Hillary Clinton. No matter who hacked or revealed the emails and data it would not make a difference. The actions in the end are the same. We should be concerned about the ethics of our leaders overall.
Who hacked the DNC emails absolutely matters, and the fact that Putin believes it doesn't should be just one more red flag. It matters because first and foremost, it is a question of national security and the fact that America could have been hacked by a foreign power is worrying. Secondly, the agenda of the hacker matters because they may have been attempting to influence the election. Transparency and sourcing are always important when considering any new information.