[Iarwain Ben-adar]

As we all know, Denethor already lost his favorite son, Boromir. Faramir was Denethors only son after that. Gandalf saved Faramir, in my opinion, because Denethor was already in such a mental state because of his constant viewings into the Palantir that another loss in the family (he had already lost Finduilas and Boromir; finduilas to grief, or of longing for the shores, and Boromir dying protecting the Hobbits). Denethor was the ruler of Minas Tirith, or Steward of Minas Tirith, and could command their armies. If he had lost Faramir, think of the commands he would have given in his lunacy! They would have lost the battle of the Pelennor because of his lunacy (ok, so maybe that is a bit of an exxageration; but he definitely would not have made things better!).

Also, people are sayng that Minas Tirith would have been leaderless if Faramir had not been saved; HELLO! Does anybody know the rightful heir to the throne, one of the main characters in the novels, the leader of the Fellowship? ARAGORN! he is the rightful king. So what if Denethor had no more descendants; Aragorn, the RIGHTFUL king, would just claim the throne as he would have even if Denethor or another Steward were ruling. It would not have made a difference if a steward were ruling or not; Aragorn would have claimed his throne after the battle of the Pelennor.

Thirdly, saving Faramir brought hope to the people. If you recall, Beregond had high praise for Faramir, and just talking about him and knowing that he would fight for Minas Tirith seemed to, to me, bring hope to Beregond.
I quote from The siege of Gondor, RoTK
"Nay", said Beregond,"this is no weather of the world. This is some device of his malice; some broil of fume from the Mountain of Fire that he sends to darken hearts and counsel.And so it doth indeed. I wish Lord Faramir would return. He would not be dismayed.

This shows that Faramir brings a certain hope to the people of Minas Tirith and Gondor because he is a fearless and noble and worhty leader. Here are the consequences if Gandalf had not saved Faramir:
-Crazy Denethor: What consequences for War and soldiers? Bad.
-Less hope for people
Those are two major consequences.All in all, Gandalf saving Faramir was a very important thing to do.