I'm not posting this as an answer because I'm not 100% myself, but the way I understand it 다면 is more of an emphasis on "hypothetically". I commonly heard 만약에 시간이 된다면 as in, "Hypothetically, if you WERE to have time..." whereas "시간이 있으면" would be more of "If you have time"
Like I said, not 100% myself but that's how I always interpreted it.