Horween is a leather company, one of the last tanneries in the US, in fact. It's been in business over 100 years and is on its 4th generation of family ownership. Horween is the official leather supplier to the NBA and NFL for game balls, and has been for decades. The company sells very high quality leather, often to custom boot and shoemakers, and apparently to watch strapmakers as well. I imagine that they charge more for Horween leather because it's some of the best quality available, and the best never comes cheap.
As far as the tangible difference between Horween supplied leather and leather from another source, that's the kind of thing you'd have to compare hands on, and preferably by seeing and working with large pieces, since that's the best way to assess overall quality.