Why does it say "la'Aretz... b'Eretz Mitzrayim... ha'Aretz"?
Ohr ha'Chayim: The food will firstly sustain Egypt. After it suffices for Egypt, it will save the land (i.e. also other lands) from extinction.
Malbim: The food gathered in the cities will be for Egypt itself. The fifth of grain gathered in the capital will be to sell to other lands, which will bring great wealth to the royal storehouse.
QUESTIONS ON RASHI
Rashi writes: "'The food shall be' - [I.e.] the gathered food." What is Rashi telling us?
Gur Aryeh: Yosef did not intend to gather all of the food to store away; for then they would die of starvation [now, during the years of plenty!]. Yosef meant that a certain percentage of the food should be gathered.
Rashi writes: "The gathered food [shall be] just like any [supply] that is stored away for the country's preservation." Why only "just like;" was it not exactly that!?