Savanna is primarily grass, sedges, and forbs with dappled large crown mature trees (typically oak). The vegetation is good for bedding deer, the trees are good sources of mast. It's what is supposed to compromise the edge between prairie and forest, but since people wiped out that transition with crops, it's missing from much of the Midwest.
The large trees sustain the understory fires which keep the other trees from filling in and shading out the grasses, etc.
It's also good for birds and small mammals as there's ample cover for nesting and dens, with plenty of bugs and seed to live off.