The manna given to the Children of Israel in the desert was a testing experience–for each individual as well as for the people as a whole.
And it was not a matter of getting used to the unusual taste and the sustenance contained in the food from heaven.
It was a matter of believing that it would keep coming.
The test was to eat all of it and save none for the morrow; it was a test of faith.
After all, how can one be sure that God's mercy extends into the future.
Many persons rely totally on the army for their safety and on the bank for their savings.
Few can rely on manna for their daily nourishment and not exhibit fear and apprehension.
Everyone receives a full portion of life each day that cannot really be extended till tomorrow.
The test is to live this with complete faith.
And from this test a person extricates the spark of holiness which lifts him to a higher level.
–Rabbi Adin Steinsaltz
From The Candle of God by Rabbi Adin Steinsaltz