First off, the things that Giles would not have been able to prevent. Willow's plan to bring Buffy back. He had no clue as to what they were planning, as was made obvious by his leaving in the first place. Likewise, Buffy's affair with Spike, for the same reason. Unless Buffy had gone to him for help as she went to Tara later, which I doubt, Giles would have been just as clueless as everyone else about how severe Buffy's state of depression had progressed, or even that she actually was depressed before "Once More With Feeling," and what it had resulted in, or he wouldn't have left Sunnydale the second time. He wouldn't have been able to prevent "Seeing Red" or Spike's leaving town in search of his soul, any more than Xander or the others could have, simply because he had no knowledge of what was happening until after the fact He couldn't have stopped Warren killing Tara, any more than Buffy could have, simply because it was such an unexpected thing.
As for what Giles actually might have been able to do, I can see two possibilities. He might have been able to help Willow in controlling her magic addiction, and he might have been able to help Buffy deal with her depression, but only after he realized that those things were happening. Weigh that against the advantage that his being able to return with the power that the Coven had given him in order to help in stopping Dark , and it seems pretty obvious that, ultimately, Giles leaving was more valuable than his staying would have been.
