There are a number of things that can drive readers away from your book, and badly-developed characters are definitely close to the top of that list. It's not always done intentionally but it's something that can ruin a good story. So take the time to learn how not to write a Mary Sue-type character.
Characters are the life breath of your novel so you need to create strong, realistic, memorable characters. You want your readers to connect with them, to cheer for them, to grieve with them...to do that, you need to make realistic, memorable characters.