In the series finale, Admiral Janeway travels back to 2378 to take Voyager back to Earth. Out-of-universe, of course, this point in time was chosen to fit in with the continuity of the rest of the series. In-universe, however, it seems that Janeway could have chosen any point in time, since she had the ability to travel to and from the Delta Quadrant at will. Is there a reason given, in-universe, for why she chose that particular incident to travel back to?
Janeway appears to have chosen 2378 for a couple of very specific reasons;
That they'd been traveling long enough that Janeway had begun to take a morally ambiguous stance toward Starfleet regulations, especially regarding the Temporal Prime Directive. She's unwilling to hear much about what happens in the future (at least at first) but with just a little convincing she's willing to fit the Voyager out with advanced technology and take on the Borg to get her crew home.
That this date represented a turning point of regret for Janeway. After making it back to the Alpha Quadrant she probably had a good long think about her actions, chief of which was her decision to reverse away from the nebula, having later (presumably a decade later) found out that she'd been mere minutes away from a portal straight back to Earth, one which would have bypassed the Fen Domar and the Borg that they would have otherwise encountered on their way back... if only they'd had some way to defend themselves against The Borg...
Note also that in another time-travel episode she was unwilling to sacrifice the Voyager's impact on the Delta Quadrant. Again, it may well be that from this point onwards that their involvement was pretty peripheral, just flying and getting shot at.
TELEK: I can assure you, Captain, that I would not do anything that might contaminate the future and perhaps harm the Romulan Empire, but, in twenty years I could alert Starfleet not to launch the mission which sent you here.
CHAKOTAY: I'm afraid that's not possible either. We've already had a huge impact on this quadrant. People and events here would be drastically affected.
Also, if she went back further (and, for example warned Starfleet not to launch the Voyager in the first place), she might endanger Seven-of-Nine's rescue from the Borg and negate the existence of Naomi Wildman, two people that she pretty obviously views as surrogate daughters.
I agree with Valorum's assertion that Admiral Janeway needed Captain Janeway to be morally flexible.
That said, morally flexible is not amoral. Admiral Janeway knew that Captain Janeway would not, for example, allow the Ocampa to die so that Voyager could use the Caretaker's Array. She needed Voyager to be in the proximity of a way home without a moral downside for her former self. The Borg Transwarp Hub offered such an opportunity. Her weapons from the future were more than enough for the Borg in the past and neither Janeway would lose sleep if a few (million) Borg died.
As for the Admiral being able to "travel to and from the Delta Quadrant at wll," it's true that Korath's device allowed her to travel to the Delta Quadrant 26 years in the past but there was no evidence on screen that the device could be adapted for a larger vessel than her shuttle. Also, she had to take a drug that was experimental in 2404. It's unlikely she would be able to get her hands on enough for the entire Voyager crew.