In Chapter 8 Moshe goes through many of the miracles that Hashem performed in the desert on behalf of the Jews. He mentions in verse 3 the Manna. He says, “Hashem made you hungry and then fed to you the manna that was not known to you or to your fathers.” Then he says something interesting. He says, “In order to let you know that not on bread alone does man survive, but on whatever Hashem dictates a man will live.”
The purpose of the manna was not to sustain the Jews in the desert because there was no other food to be had. Hashem could have provided for the Jews differently, as he did with the quail or bring some other seemingly natural type of food.
Moshe is telling Bnei Yisrael that when Hashem sent them the Manna, it was a means to an end. It was meant not just to sustain them but to teach them a lesson. To teach them that Hashem can, and will, provide for their needs.
As the Jews are about to go into Israel, they are at risk of forgetting all these lessons they had experienced in the desert. The desert was a magical place for them. All their needs were taken care of. They saw miracles openly.
As they are about to enter Israel, they are also about to lose that lifestyle. They are now going to have to begin working for their food and living basically normal lives. They could fall into the trap of thinking they are providing for themselves and they are earning their own way.. That is why Moshe says all this now. To remind them of who is really behind it all. That is why he continues by saying that Hashem is bringing you to the land and you will eat and be satisfied and bless Him for the good land that He has given you. And then he continues to warn them from falling into the trap of saying, “Kochi V’Otzem yadi” – that their accomplishments are because of their own efforts and they provided for themselves.
Remember the manna and all the other miracles and you will see that Hashem is really behind all the sustenance you have.