who were messing with stuff they shouldn't have been messing with, just like MIB was... and who were warned not to do it, but were driven to experiment at all costs (even willing to kill the natives). Their story explains what Fake Mother was worried about - when certain people get exposed to the power of the light they lose their perspective. It was very realistic, exemplified the 'it always ends the same' 'rule' and showed why the Other's decided they had to go. It was a brilliant way to humanize them and justify the purge as self-defense.
It was the main theme of Lost (and of Jacob's test) that anyone could go either way at any point depending on their free will decisions. It wasn't what side you were on that made you the 'good guys', it was the decisions you made individually. By humanizing everyone (except maybe Keemy) we get to see the individual battles are in the heart and head, not against some 'other side' evil.
Ben exemplifies this going back and forth the best... He was the most evil, but they humanized him which made it totally believable that he could come around in the end.
Same thing with MIB - they humanized him and made his story so much better by removing the idea of him always being evil. He made choices that led him that way and he befell circumstances beyond his control that pushed him that way. Could he have turned around and not been evil anymore like Ben? That is a serious question that only comes to light after Across the Sea.
If they left Dharma mysterious we wouldn't get that same dynamic that they gave us in the rest of the show with almost all the other characters (again, except Keemy *hehe*)