This question already has an answer here:

Between the return of Lord Voldemort at the end of book 4, Goblet of Fire, and the end of the series, how was Snape useful for the Order of the Phoenix in fighting against Voldemort?

merged by Null Mar 17 '16 at 13:59

This question was merged with What good did Snape do anyone by infiltrating the Death Eaters? because it is an exact duplicate of that question.