2 3 questions for those who subscribe to flat earth theory:

There is no “flat earth theory” in a scientific context (nor could there ever be, just like there isn’t and can’t be a “round earth theory”). Even in the colloquial [incorrect] definition of the word theory, there is no singular or unified “flat earth theory” of which to speak. Most every earnest flat earth researcher has their own particular/unique view.

I would say, “flat earth posit” to be more precise.

1. What do you believe the motive is for the lie? and

What lie? Humanity is merely stupid and wrong, as it historically always is.

There may be some liars in the group who push a particular view for personal gain (the reason people lie), but the vast majority simply repeat what they were taught. For an example, most aren’t lying when they say “the vaxx saved the world from covid and got us back to normal”, they are just wrong.

2. Even if you're right, what do you believe would be the benefit of everyone learning the truth about it? How would the world knowing the truth benefit from this knowledge?

A very common question, with many possible answers and perspectives. I’ll provide my answer, but you may want to consider also asking these questions over on flatearthresearch to hear some of the others.

Assuming someone knows better and is lying about it (a large and unnecessary assumption), the reason is almost certainly topography/cartography and control of the slave populous [citizenry]. What did many europeans choose to do once they learned the maps they had were wrong and there was a “new world” available to escape their hideous lots?

On the other hand, if people are just wrong - as is my view - then one benefit is the same as above (the potential ability to go somewhere else and escape the tyranny/slavery), and another is correctly understanding reality better / discarding incorrect mythology masquerading as science (which both pay extreme dividends in countless ways).

3. How would you rank the importance of knowing this truth compared to other truths that have been dubbed "conspiracy theories?"

“Knowing this truth” is certainly important, but not nearly as important as recognizing you could be wrong/ignorant and pursuing truth (writ large). Flat earth research is perhaps the most valuable and rewarding subject that exists to study (and in so doing, learn how/refine your ability to study and think independently), which is fundamentally why there is such an extremely well advertised (i.e. funded) psyop to suppress and ridicule it. Counterintuitively, It has very little to do with the shape of the earth.

Why spend your energy trying to convince your fellow man of this theory as opposed to others?

You shouldn’t spend you time trying to convince [manipulate] others of anything. Like argument, another form of manipulation, it is a fools errand.

Instead you should try to share your views and genuinely help encourage/foster/kindle curiosity.

I encourage you to ask as many such questions as you can/want, as long as you are earnestly interested in the answers/discussion! Flatearthresearch might also be a good place to ask such questions.