Why did Gandalf save Faramir?

[Guinevere]

Well, already when Denethor sends Faramir out on the hopeless errand to hold the passage through Osgiliath against the enemy, Gandalf tells him:

"Do not throw you life away rashly or in bitterness." "You will be needed here for other things than war."

So he clearly thought, that Faramir was important for Gondor. He is not only noble (Somewhere it is stated that "the blood of Numenor runs more true in him than in Denethor or Boromir" but I can't find the quote right now.) but he is also wise and compassionate, and the people of Minas Tirith love him.

And, like Bricho said, Gandalf was the only one who could save Faramir.

Well, I must come, since no other help can reach him.