What advantage is there in eating old produce?
Rashi: This is because the old crops in the storehouse will mature and improve with the passing of time, to the point that grain that is three years old tastes better than last years' grain.
Seforno: The number of Nochri farmers and vine-growers to gather the harvest will decrease, so what remains from year's harvest will suffice, together with the meager amount gathered this year; you will be sustained without toil. 1
Why is removing the old crops from the storehouses a Berachah?
Rashi: Because the granaries containing the new grain are bursting to the seams, and one needs to clear the old grain out of the storehouses to make way for the new grain.
Rashbam: They will remove the excess produce from the storehouses in order to sell it (Seforno - to other countries). 1
Moshav Zekenim: The first and last letters of "Mipnei Chadash" spell Mashi'ach, and [with the next word "Totzi'u"] the last letters spell the name of a false Mashi'ach that the masses followed; they will cease to believe in him.
QUESTIONS ON RASHI
Rashi writes that the old is better than the new, and so it says in Bava Basra 91b. Why will you take out the old, in order to replace it with something inferior?
Mizrachi: The storehouse guards new grain from rotting. You will clear the old grain from it to elsewhere.
Shir Ma'on (5) said that you will sell the excess food to far away countries. It is better to sell old, and put the new into the storehouses to age! (PF)