Why did they save food in the cities, and not in the fields?
Moshav Zekenim: In famine years there are many thieves, and food would not be secure in the field.
Why does the verse say, "v'Yikbetzu Es Kol Ochel" and "v'Yitzberu Var"?
Malbim: They will save all food (not only grain), e.g. legumes and fruits. Surely all kinds of foods will grow in the years of plenty. This will be gathered bit by bit. The grain will be gathered much at once, under Pharaoh's jurisdiction, in the royal storehouse in the capital; the Chacham that Pharaoh picks will oversee it. The [other] food, lesser officers will oversee it in cities, and guard it [from theft].
Ha'amek Davar: In years of plenty, people eat grain and discard Ochel, i.e. foods such as vegetables that cannot be saved (without rotting). Now they will gather Ochel to feed it to people in the years of plenty, and they will store the grain and guard it.
