What is the 的 doing in 他很快就会回来的? I came across this simple sentence:

He will be back soon.
他很快就会回来的。 

I understand that the 就 is expressing earliness, but what is the 的 doing here? In the textbook 新的中国 it doesn't say 的 is part of the 就 construction, or on the allgrammar wiki here https://resources.allsetlearning.com/chinese/grammar/Expressing_earliness_with_%22jiu%22

Is 的 turning the 会回来 into a sort of adjective? Since 会 is an adverb I expected it to be followed only be the verb 回 with the directional complement 来 without the 的. Any help would be appreciated.
As a native Chinese speaker, I seldom hear people say"他很快就会回来。" 
Because without the word "的" it sounds weird and unnatural.
There is no meaning of “的” in the sentence.
I think “的" use in the ending of this sentence is just make the sentence better.