Some parts of the carcass like the brain, internal organs and bone marrow contain precious fats, in short supply on the savanna. In particular, bone marrow contains long chain fatty acids that are important for brain growth and development. Once meat was a regular part of the diet, it could be bulked out with raw calories from any source. Little would have been available in the dry landscape inhabited by Homo erectus . The kinds of plants that survive in these conditions often produce underground tubers as storage organs that cache their surplus energy as starch.
Tubers are good energy foods, but have little other nutritional value.
This doesn't matter if meat was available to provide the protein and minerals.