It also acts as a test case. "Hey seed-generator, give me a random number between 0 and a gazillion." "Does 4 sound good?" "No, it sounds like you're obviously broken."

In theory, all values would be equally random. In practice, some results are exceedingly more likely to occur when something's wrong with the algorithm.

While any given outcome of flipping a coin randomly N times is equally likely, if that series looks like [heads, heads, heads, heads, ...], then there's probably something other than pure random affecting the output.