I wonder if both sentences are different. Can anyone tell me the difference between 就要。。。了 and 要。。。了?
In practice, I doubt if anyone really says 明天他要回来了 although there seems to be no grammar mistakes in the sentence. 明天他就要回来了 or 明天他就回来了 is more likely to be used colloquially.
I think it probably because the way Chinese expresses this is different from English. If we want just to express "he will be back tomorrow", we would say 他明天回来.
"就" in "X 就 Y" denotes " when the condition X is met, the occurrence Y will happen right away"
In "明天他就要回来了" the condition is "when tomorrow arrive" and the occurrence is "he will come back"
明天他要回来了 --> he will come back tomorrow
明天他(就)要回来了 --> he will come back (right away) tomorrow
meaning "he will come back immediately or shortly after tomorrow arrives"