Cats make this strange sound (of which there are many variations) because they are hungry and they are enjoying their food. There is also an element of being defensive of their food by which I mean they are sending a signal to others to keep away while I eat. This means you’ll hear it more when small/medium wild cats eat. They have to defend their ‘prey’ with more vigor than domestic cats.
The sound is a modified cat version of the sound that humans make when they are hungry and enjoying themselves eating. Under these circumstances we indicate that we are enjoying ourselves by making an “uummm” sound. Cats make the “nom nom nom” sound; not much difference when you think about it.
If you have an alternative explanation please share it. We can discuss things.