A stomach built for variety
Like cattle and sheep, goats are ruminants — they have a four-chambered stomach (rumen, reticulum, omasum, abomasum) that lets them digest plant cellulose no human or single-stomached animal could break down. The rumen is essentially a fermentation vat: billions of bacteria, protozoa, and fungi break down tough plant fiber before it ever reaches the goat's own digestive enzymes.
What sets goats apart from cattle and sheep is behavior, not just anatomy. Cattle and sheep are grazers — they put their heads down and eat grass close to the ground. Goats are browsers, evolved to reach up and select leaves, shoots, bark, and buds from shrubs and trees, the same instinct that lets them work standing on their hind legs to reach a blackberry cane or a low tree limb.
Handling the plants other animals avoid
Many brush and weed species defend themselves chemically — with tannins, alkaloids, or bitter compounds that make them unpalatable or mildly toxic to grazers. Goats have specialized liver enzymes and salivary proteins that neutralize a meaningful share of these plant tannins, letting them eat species like blackberry, poison oak, and scotch broom that cattle and sheep largely leave alone.
This is also why goats are considered "goats," not sheep with an image problem: the browsing instinct and the tannin tolerance work together. A herd will typically clear the woody, bitter, high canopy layer first and leave finer grasses for last — the opposite order sheep prefer, which is part of why the two species are often run together on the same property for complete coverage.