Why Celestia Is Useless

A fan theory on why the immortal princess of Equestria never chooses to confront world-ending threats directly despite her obvious power to do so:

In the time before Celestia, the unicorn magi of old were responsible for the raising of the sun and the moon. From what we know, this required enormous power and was only performed by the most senior and gifted of the unicorns. However, the strain was so great that even these vaunted masters of the arcane were unable to participate more than a few times before risking permanent magical burnout. Celestia and her sister are unique in their ability to sustain the task indefinitely at no harm to themselves.

It can be inferred that, as with most inventions, the old way was eventually forgotten as ponies had no more use of the complex spells and training needed to move the sun. It is very likely that few, if any, still remain in the modern world who would be capable of the task if required. As such, Celestia must consider herself and her sister as the most valuable assets in Equestria, perhaps even the entire world. Such assets are too valuable, and their potential loss too devastating, to be risked in open combat. Thus, Celestia must remain as far removed from the conflict as she can, for despite her longevity she is still fundamentally mortal and can be maimed or killed.

However, we have also seen that Twilight Sparkle, once gifted with the appropriate magics, was more than capable of changing the day from the night (albeit with humorous results). From this we can infer that Celestia imparts the largely forgotten knowledge of celestial movement into her most trusted students. This is most likely as a safeguard against the extremely possible event of her own death. Obviously, this knowledge is largely restricted out of necessity lest absolute chaos descend as multiple factions vie for total control of day and night.

This has been a fan theory on why Celestia is useless.