Arpaio is a free man and will never have to answer for anything related to the charges that were brought against him.
President Trump's pardon, however, was issued after Arpaio's legal team had filed a motion to have the conviction overturned. This case is still open on the judge's docket, and the judge has asked the U.S. Justice Dept. (the prosecutors in the original case against him) to file some kind of response to Arpaio's motion. If the Justice Dept. doesn't file a response, Arpaio's conviction might be overturned anyway -- which makes the pardon a moot point.
Yeah it sounds like it’s just a paper-work drill to officially close out all proceedings that were already initiated.