it began in the old days because there really was no way to inform the masses about the candidates, and the majority of the people didnt know who to vote for. so the idea was, have joe farmer would vote for an elector he KNEW, and then trust that elector with the job of making the decision of voting for the better candidate.
nowadays, our states just have everyone vote for the president, because everyone pretty much knows about the candidates. almost all of the states (except colorado) have a system wherein the candidate with the most votes picks all of the electors, who (in theory) will vote for him. now, its always possible an elector could vote the other way if they wanted to, but the candidate usually picks extremely trustworthy people.