"for" Why? That's a tough question. "Could you do something for me?" "Why did you do that to me?" Perhaps "for" is more often used to mean that the object will receive a benefit and "to" to mean that the object is receiving something communicated by the subject. This is my own created answer and I am sure is incomplete and there may be some standard explanations in grammar books. I just thought of another example: "it is important to / for me to improve my skills" Here, "to" makes the phrase mean that I value the idea of improving my skills. "For" makes the phrase mean that I need to value my skills for some purpose. I am sure I could continue and reach a better definition of the distinctions but I hope this is a good start.

"It would be great for me" because using 'great to me' doesn't make sense. If something was 'good to me,' that usually indicates it treated you well and was nice, friendly, or enjoyable (She is good to me = She is nice to me). 'Good/great for me' usually indicates something that is beneficial to your health or well-being (She is good for me = She beneficial to me as a person, such as my health or self-confidence, or well-being).