It's the only way to connect the Byakugan to the Three Doujutsu. Not necessarily connect the Hyuuga to the strong bloodline of Rikudou but the byakugan..
One can still see it as fan service but may I say a relevant one.
Kaguya having Byakugan would explain how and why she came to eat the fruit. Because it is made of chakra and she saw it.
Well you can also call it closing plot holes. Kakashi in part one state that the Byakugan and the Sharingan have some sort of connection.
Right now it seems like the Byakugan was the inborn ability of Kaguya, she got the third eye (Rinnegan/Sharingan crossover) through eating the fruit and the "original" Byakugan was transformed into what it is now.
Furthermore this gives more evidence towards the theories that the Sage's brother is the direct ancestor of the Hyuuga (and possible Uzumaki) Clan(s).
Not realy.. something that actually makes sense. Only problem now is...... where they hell did the hyuuga come from?
Kaguya had the Byakugan (likely even before she ate the fruit), so the Hyuuga must be directly related to her. We only know of two sons, Hagomoro gave birth to the Founding Fathers of Senju and Uchiha. Conclusion: the Hyuuga and possible the Uzumaki come from his brother.
If you want to know about Kaguya's origins and why she had the Byakugan, well she doesn't look really "human" so my guess is she is either demon, a goddess or another species. So the "original" Byakugan maybe was an inherent ability of her race. As she ate the fruit it delevoped into the Byakugan we know now.
@TheRapGuy:
There is a difference between the Byakugan she had before and after eating the fruit, the "new" Byakugan needs chakra the first one likely not. The abilities before maybe were not as developed or strong and were enhanced from having chakra.
Where was it said that there were two Byakugans or that it was affected by the fruit? Either I missed it or you assumed it was like this.