In The Clone Wars season 6, Qui-Gon Jinn told Yoda that he was chosen to learn how to retain his consciousness after death. He was brought to the Force Priestesses to pass the trials before his training could begin. There, when the Priestesses were debating among themselves why he should be trained, Serenity mentioned that "his destiny was already set, it is not for us to decide...He is to teach one who has to save the universe from a great imbalance. For this, the great gift will be his".
I assume that this is in reference to his future role in training Luke Skywalker to become a Jedi, so that Luke may play his part in the defeat of the Sith, thus bringing the Force back into balance. I would also assume that the "gift" of consciousness after death was granted to enable Yoda to fulfil his destiny, not merely as a reward for success. Promising Yoda with greater power would be counter-intuitive of the Force, since that is the way of the dark side, the entity in question that the Force is trying to bring into balance here. It would also have been unnecessary for setting Yoda down this path - something like a vision would have sufficed.
Based on those assumptions then, why is the ability to retain consciousness after death necessary? When Luke returned to Yoda before his death, Yoda told him "No more training do you require." So I assume Yoda wouldn't appear before Luke after dying to give further training, in the maximum possible span of a few months between his death and the Battle of Endor. I doubt Yoda was taught this ability just so he could pass it on to Obi-Wan, the one person who actively used his retained consciousness to guide Luke. This reason sounds too roundabout to me, not to mention that it was Qui-Gon, not Yoda, who trained Obi-Wan during his exile. If Obi-Wan was the intended beneficiary, and he was strong enough in the Force to communicate with Qui-Gon during his exile, including Yoda in this is completely superfluous.
So, exactly how was the gift of retaining consciousness after death intended to serve Yoda's destiny of training one to save the universe from this great imbalance? Or am I barking up the wrong tree entirely, and Yoda's destiny spoken of by the Force Priestesses is to make a comeback in Episodes VII-IX, a time when there are still new Jedi to be trained and when Yoda is well and truly dead?