Fully developing your backstories always pays off. Not only do they contribute to the quality of your characters and plot; you can always offer them as freebies to potential readers. Some background information can be referred to in your main story, but including too much can bog it down. Some can be dropped in here and there as flashbacks. However, lengthy explanations of past experiences that influenced the character aren’t always relevant to the story at hand. Nonetheless, if they’re interesting you can use them as a short story/teaser to readers who visit your website or sign up for your newsletter.