There is a legend behind this festival as there is one after every story. In short, the story goes like this:
Once upon a time was a queen called Veeravati who was very delicate disposition thanks to the pampering by seven brothers. Being the only sister, they loved her very much. She was given to a king (naturally) in the marriage. After being married, queen went back to her parents, to celebrate, the festival in which women has to be on fast whole day breaking it when the moon shows on the sky.
Due to the hot weather and her delicate being, she started having hunger pangs and was on the verge of passing out by evening. Her brothers could not see her plight and concocted a farce by making a false moon behind a hillock and thus forcing her to break the fast.
But she earned a curse for this from Shiva, but being very truthful and faithful, Shiva agreed revoke the death of her husband but he would live in great pain with needles embedded in his body. This then happened but queen, would remove the needles one by one. At last, one day when she had gone out, the maid removed the last needle bring a great succour to king who took her as his consort and queen made the maid instead.
But she did not lose heart and had great faith in her devotion and love and things moved in such a way that king realised his mistake and made her his queen again. What happened to maid is anybody’s guess.
So this fast is to be completed otherwise bad things can happen to the husband (as if it is not bad enough being married). There are some foolish men who I think do this for TV coverage who also observe the fast alongwith to show the undying love for spouse. For this they must be bunking the office work to be with the womenfolk all the day. Maids can be afforded by the rich people, so this means this ritual is applicable to the people who are affluent. For the poor, it is daily Karva chauth one way or the other with the difference they have to worry about how to break the fast.