"And the benei yisrael saw [it] and they said to each other "man hu ['its food!' or 'what is it!?']" as they did not know what it was, and Moshe said this is the bread that Hashem has given you to eat.(Shemos 16:15)
"And the nation rested on the seventh day. And the House of Israel named it "man" and it was like a white gad seed, and it tasted like honey pastry." (16:30-31)
The "dor deah", the desert generation that is forever revered for the intimacy which they were privileged to have with Hashem, became entrusted with the task of naming their food, gifted daily from Hashem Himself. Surely you and I would have thought of something clever that would speak of the gift that it was, the love that it expressed, the parnassah that it assured or the nourishment that it provided. Yet this generation who had reached the highest levels of knowledge could find no more a creative label than "man". Despite its pearly presentation, honey like sweetness, and fresh as dew packaging, they called it "food" according to Rashi or "whatchamacallit" according to Rashbam and Ohr Hachayim.
Furthermore, the sequence of events requires comment as well. Why does the Torah choose to record the name of this desert food immediately after the Torah mentions that we observed shabbos?
Perhaps the "dor deah" understood that we can get used to almost anything, even food appearing out of thin air, every day, in time to feed each family and restock the empty shelves of all homes. Accordingly, in a possible attempt to protect us from taking the "man" for granted, they tried to capture for all times to come, the excitement of those first few moments when it was new and totally unexpected. It was just then that "they said to each other 'man hu' ", which the K'sav sofer interprets as each one wanting so share his unique "man" experience, its singular taste and no doubt some personal insight as well. "Man" will forever remind us of "man hu" and those first few moments when they had no words to fully express or describe what they saw.
It is also quite possible that only after they had lost the "man" for a shabbos, that they come to appreciate it anew and ponder life without it. They then may have felt the urgency to establish appropriate reminders to assure that we would never become jaded about all that Hashem provides for us.