"It's a convenient thing," said the unicorn.
Twilight turned to look at him. "What is?"
"Harmony," he replied. "Something you know is good. Something beyond you, tangibly there, that you know is right."
Twilight smiled at him. "I take it there were no Elements of Harmony in your world?"
"None," he said. "Nothing like them, either. It would have been nice to have a light like that shining on the right path."
Twilight still smiled. "Harmony is more than just its power or its manifestation. It's about being with others, together. Even without its magic, there's still a point in being united and not divided. I'm sure your ponies knew it too, if they lived and worked together as we do here."
"The purely factual benefits of a collaboration between different individuals aren't something that needs a magical tree to be highlighted," said the stallion. "But it's still useful to have something like that tree. To have something higher and indisputably good."
Twilight frowned slightly. "Is ponies being in harmony with each other not indisputably good, regardless of whether Harmony is there or not?"
"It is, I suppose," the unicorn replied. "That's not the point. The point is that you have a frame of reference. Something you know for a fact is good, something you can measure against when trying to determine if something else is good. Something beyond you to be held in high regard."
"I'm still not sure I get it," said Twilight. "Is harmony across ponies not beyond the ponies themselves?"
"As an ideal, perhaps. And many ponies can together choose to follow a common ideal," said the unicorn. "But it's a formal agreement. Not the logical consequence of hard evidence. Without something above us to measure against, good and bad become what we choose and agree them to be. Without an indisputable source, any pony is able to choose for themself what they believe is good. So it's convenient for you to have Harmony. It's useful."