Browsers and grazers
Goats are browsers: like deer, they prefer leaves, shoots, and woody brush, and will turn to grass only later. Sheep are grazers: they eat with their heads down, taking grasses and low forbs first. That single behavioral difference drives which animal fits which job.
In open oak woodland with abundant grass, sheep do most of the work. In dense forest or brushy undergrowth, goats are the better tool.
Why combine them
Because their diets complement each other, operators frequently run sheep and goats together: the goats knock down brush and invasive shrubs while the sheep clean up grass. On a mixed property, that combination clears a wider range of fuel than either species alone.
It’s a practical reason to ask an operator about herd composition — the mix should match your vegetation.