In spoken English, people don't usually say, "you are important for me" because it feels like that person exists to serve a purpose solely to the speaker.  We use to and it makes the importance more subjective.  For example, to your wife, you are important because you're her husband and her son's father.  To strangers, you're not important because they don't know you and have no relationship with you. 

More examples:

- This ring is important to me because my great grandmother gave it to me. 

- This love letter is no longer important to me because we broke up.