Phones sold by carriers are sometimes locked to a carrier's network, meaning it might not work when customers switch to a rival or get a separate phone plan when travelling abroad.
Another advantage with having an unlocked version: The phone won't come cluttered with carrier-specific apps, many duplicating functions available elsewhere.
Samsung is selling the 5.8-inch S8 phone for USD 725 and the 6.2-inch S8 Plus for USD 825. Both prices are about USD 25 cheaper than what the four main US carriers charge.