G-d told Moses [to tell the people about the manna...] "The people will go out and gather each day's portion on that day."
Even if we believe that everything is in G-d's hand, we still tend to think that our own efforts also play a role in acquiring our physical sustenance. In contrast, manna was not acquired through human effort, and so left no room for such misconceptions.
Even so, G-d did not allow the people to collect more than one day's worth of manna at a time, for whenever the pantry would have been full, the people would not have felt dependent upon G-d.
On the other hand, G-d did require the people to go out and gather the manna, rather than deliver it to their doorsteps. In this way, He prepared them for their eventual entry into the real world. If acquiring the manna had not required any human effort, the people would have dismissed it as an isolated miracle, irrelevant to real life. By being required to collect the manna, they learned that human effort and G-d's blessings work together.
The manna taught us that our sustenance comes from heaven. Even when it appears to be the fruit of our own labor, it is in fact a gift from G-d.
--Excerpt from Daily Wisdom