If you're curious, The Secret Life of Groceries is a good (and interesting) read.

One tl; dr thing is that the supermarkets are a _supplier_ to both end customers and those with products to sell. They supply shelf space and customer reach, in the very literal sense that companies bid for things like endcap placement (the end of the aisle being better than being in the aisle, since everyone making an orbit through, say, the deli section will pass your goods).