The lechem hapanim was a specially shaped form of matzah – for all the bread (with the exception of the shtey halechem to be discussed) used in connection with Temple serviceswas unleavened. It was placed on the shulchan [a shelved table] in the mishkan and the Mikdash every Friday and consumed by the kohanim on the rotation as soon as the batch was replaced the following week. Though the lechem hapanim was a form and so was manmade, it was sustained by a miracle (not preservatives) that kept it fresh and warm for the duration of an entire week. So the lechem hapanim epitomizes a a synthesis of human and Divine endeavor.
Recognizing that form of partnerhsip is what offerings are all about. Though we know that G-d the Creator is the ultimate Maker of everything, what we offer are things that we have worked on. G-d gives us the raw materials, and human beings add value through labor, working to cultivate crops, domesticate animals, and then complete all the tasks inherent in food preparation.
Bread in particular epitomizes human creativity. See http://kallahmagazine.blogspot.com/2012/01/natural-vs-man-made.html. It doesn't grow ready like raw wheat; it requires human labor to grow, gather winnow, and grind the grain. Then one has to knead it into a dough and bake it to finally produce bread.
What we do in making our offerings to provide sustenance for the kohanim is that we show that we recognize that even what appears to be the fruit of our own labor is due to what is granted us by G-d. It is the opposite of the assertion of kochi veotzem yadi asa li eth hachayil haze. That's why the 3 regalim are also harvest festivals. We come with the joint product of G-d's natural products and our manmade one as we break bread with G-d.