Galadriel's intentions.

[Halbarad]

Boromir was correct in saying that Galadriel offered that which she did not have the power to give. All they had to do to get what they desired was

to turn aside from the road and leave the Quest

. I think her reasons for doing this were to temper the individuals, to make them realise why they were really there. Better to do it while they're in a safe place, for they get to think about the choice and make a decision without doing it at a time or in a place where it could jeopardise the Quest. A chain is only as strong as its weakest link, and if she could get the link to crack early, so much the better.