Origin and Significance
Once the bride and this woman had become god-friends or god-sisters, they would remain so all their lives and recognize the relation as such. They would also treat each other like real sisters.
Later in life, if she faces any difficulty related to her husband or in-laws, she would be able to confidently talk or seek help from each other. Thus, Karwa Chauth was started to as a festival to celebrate this relationship between the once-brides and their god-friends (god-sisters). Fasting and praying for husband came later and is secondary. It was probably added, along with other mythical tales, to enhance the festival. The husband would always be associated with this festival, because the day of starting this holy friendship between two god-sisters was essentially the day of bride's marriage to him. Thus, praying and fasting for him by his wife during a celebration of her relationship with the god-friend would be quite logical.
Hence, the festival of Karwa Chauth was to renew and celebrate the relationship between god-friends (god-sisters). It had a tremendous social and cultural significance when world was not having the way to communicate and move around easily.