While I thought that Federer was comfortably the greatest player in the history of the open era anyway before this title win, this latest success his pretty huge for his legacy.
It significantly enhances his longevity credentials, with a 13 and a half year span before his 1st and 18th grand slam title wins, an open era record. After his 30th birthday title he has now won 2 slams, 1 YEC, 7 masters series titles and 22 titles in total.
Also he adds a 'heroic', completely unexpected, 'against all odds' grand slam title win to his collection, at the age of 35, after spending 6 months injured on the sidelines, coming through 3 five setters and beating his ultimate nemesis and rival in the final. That was after he beat the guy who had won the most recent slam in the semis. Plus many people including me had completely written him off and thought this ship had sailed after all those slam final defeats to Djokovic in 2014-2015.