Why does the Torah see fit to add this Pasuk, having already informed us how the two pillars led Yisrael by day and by night?
Rashi: To teach us that, after the pillar of fire arrived, the pillar of cloud remained for a short while before departing,
Oznayim la'Torah: So that Yiisrael should not be for even one moment withouht Divine protection - See Oznayim la'Torah. Mechilta: And it is from here that we learn that one should begin to bring in Shabbos (a short time) before nightfall.
See Torah Temimah, note 80, who elaborates.