I'll quote GOP rep Darrell Issa here, in his reasoning why there needs to be a special prosecutor and investigation. This is from his interview with Bill Maher recently:

“You’re right that you cannot have somebody, a friend of mine, Jeff Sessions, who was on the campaign and who was an appointee,” Issa told Maher. “You’re going to need to use the special prosecutor’s statute and office.

"Here’s the reason we’re going to have to do it, Bill,” the congressman said. “There may or may not be fault, but the American people are beginning to understand that Putin murders his enemies sometimes right in front of the Kremlin ... he’s murdered people and taken down using cyber warfare in Georgia and Ukraine.

“This is a bad guy who murders people. Who runs a gas station with an economy the size of Italy but is screwing up things all over the world, that we’ve been, quote, ‘working with.’

“Now, we have to work with them. We don’t have to trust them. And we need to investigate their activities, and we need to do it because they are bad people.”