Having already informed us how the two pillars led Yisrael by day and by night, why does the Torah add this Pasuk,?
Rashi: To teach us that, after the pillar of fire arrived, the pillar of cloud remained for a short while before departing, 1 and vice-versa, 2
Oznayim la'Torah: So that Yisrael should not be for even one moment without 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.