|Art: Vladimir Kush|
The manna reminds us that a Jew's food, and his livelihood in general, come directly from G-d, in a manner of "bread from heaven", i.e. even though, at first glance, it appears that a person earns his livelihood through hard work, in truth however the work of his hands is merely a "receptacle" into which G-d places his blessings.
In other words, it is not the hard work in itself that brings a person his daily bread - G-d provides a Jew with food in a manner which is not limited to the rule of nature. But in order not to disturb the natural order which He created, G-d garbs His gift in natural phenomena, so that it should appear to come from nature alone.
The eternal perpetuation of manna reminds us that, even though we no longer see bread coming from heaven, nevertheless, in truth the bread continues to come from heaven to this day.
Source: Sichas Shabbos Parshas Beshalach 5751, Lubavitcher Rebbe