If I understand you correctly, you want to know what purpose you think Mary Warren could serve.  If this is the case, Danforth smells a rat, and he knows that Mary is probably the weakest of the girls.  He hopes that she will serve up a confession, but unfortunately, Danforth is not terribly strong.  His understanding of the situation is purely legal; he does not have a clue about the underlying mob-like psychology that has swept through the ranks of the girls and through the town.  He confronts truth and lies but doesn't have the will or the brainpower to make a distinction between either one.  His solution is to simply give up.