Future Simple or Future Continuous

Hi, I'm confused when to use the future simple and the future continuous tenses. I have done an exercise and my teacher told me my answer was wrong:-

Question: "Do you want me to give James the File?"
My Answer: "No, it's ok I'll see him this afternoon so I can give it to him then."
My teachers answer: "No, it's ok I'll be seeing him this afternoon so I can give it to him then."

Can anyone please tell me why my answer is not correct and what's the difference between these two tenses?

Thanks!

There is nothing wrong with your answer, other than there should be either a comma or a full stop after "it's OK" (and OK should be in upper case). There is no need to capitalise "file".
"I'll be seeing him later" is a little more colloquial and it's not necessarily a version I would teach students until they are at least upper intermediate. I would give "I'll see him later" or "I'm going to see him later".