The difference is often misunderstood but is essential when considering environment and use. The main advantage in a blend is the smooth, even appearance of the resulting turf. Ryegrass is popular because it grows quickly and is uniform and quite beautiful. Disease resistance is another trait of this popular grass. Ryegrass cultivars have a generally good resistance to brown patching, and leaf spotting.
Ryegrass mowing qualities, heat and cold resistance are also very good.
Blending various cultivars together takes advantage of individual traits in combination to produce a better overall lawn than any single cultivar could. Since the differences between grasses of the same species are relatively minor, the effects of possible trait combinations can be noted as small variations. The obvious question is if blending cultivars of a species makes for a somewhat better lawn, why not mix species and take advantage of the larger differences. That is the entire concept behind grass mixes. An example that makes the point is the popular mix of ryegrass and bluegrass.