The Others are Jacob's "Knights."
Their purpose is to assist Jacob, and that means using the Island as a conveyance in order to subtely influence history and bring about a 'convergence' of mankind.
They're chosen for being 'good.' Since good is not defined for us in the context of the show, I've looked at it as a term that refers to the fact that they have let go of their personal demons and real-world attachments, and become monastic in outlook as a result (see: Eko and the fact that he was on the list, the fact that Goodwin says that Ana-Lucia could become 'a good person,' and the telling fact that Jack wasn't on their list). No attachments, no desires, nothing except serving the will of Jacob/the Island.
Maintaining this asthetic purity is near-impossible in our 'real' world - imagine how much harder it would be to resist temptation if you had an Island that functioned (in small, petty, part) like Biff's stats book from BttF II. I imagine this is why Widmore's sins are so catastrophic, from Ben's perspective. He's violated the monastic rules and to someone comparably 'fresh' and willing to devote himself totally to Jacob (as early-Ben seems to be) it's not just a violation of the rules, it's a betrayal of the brother/sisterhood that the Others represents.
But Widmore's sins are also, to an extent, understandable. If I lived this way on the Island, saw my life passing me by, realising that I'd never make a mark on the outside world, that the 'real' world would never know me or recognize the contributions I had made to its essential progress and well-being, I believe I'd be tempted to create a secret legacy (his off-Island child/children) and empire as well.
This temptation may help to explain the pregnancy plague as well. If Jacob saw it as obstructing his goals, he'd use whatever ability/power/device the Island harbors to stop the babymaking. The Island is, after all, a quantum 'magic box' that grants wishes. Jacob's will alone, if we buy into the Island as a 'possibility generator,' could keep Richard young and keep women from birthing - to further his plans for the world.
Of course, we still don't know if those plans were for man's 'betterment.' What a switch it'll be if the MiB ends up the figurative 'hero' between the two of them.
I think the 'regular Others' perhaps aren't told the full picture any more than the 815ers or we, the audience, are. They're not told 'We're doing this because of *Insert Jacob's ultimate plan here*.' They're told 'you're doing this, because I'm paying you/because I brought you here.' As example: "Go get that weird kid who can summon birds," "Go arrange for a bus to hit Juliet's ex-husband. Then recruit her." If they ask why, I suspect the answer is always the same: "Because Jacob says so."
The leader/high priest dictates the word of God/Jacob, and the people/Others follow that word. You can see that the regular Others aren't totally happy with this situation from the way that they react to Anti-Locke's rabble rousing in the season five finale. Devoting yoruself to a sect that demands utter faith is intensely difficult. It's difficult if it's a 'religion' in our real world sense of the word, and it's (I imagine) just as difficult when there may be an actual 'God' involved. Both instances rely completely on faith, not proof.
I wouldn't be surprised if the power structure of the Others essentially mirrors both the power structure of Dharma and the 815ers - in those instances, as appears to be the case with the Others, a few people in the 'circle of trust' were given information considered sensitive. The vast majority though, were there to cook, to clean, to mend...in other words, to serve those in 'real' power.