Hey, favoriting is different from Retweet, but it's not useless. :)
So someone does like your tweet and would want to express that, but either s/he are too choosy about what s/he wants to retweet, or may be your tweet is something s/he likes, but thinks that it wouldn't be relevant for their followers.
Sometimes, people have multiple twitter accounts, because they do not want to mix their followers who are interested in extremely different things.
So that's the point of favoriting. :) I like your tweet, and would save it in my favorite tweets for future reference, but either I don't like it enough to retweet, or my followers will bang my head for crowding their feed with irrelevant stuff.
By the way, with the change in policies and execution in Twitter off late, favorites will have a lot more advantage than they did earlier. They will have more visibility in people's manipulated (by Twitter) timelines probably. So let a highlighted heart make you smile, and don't think, "Why not the circular arrows instead?" :D