It's also rather amusing and bent, and there's pretty pictures, so don't stop here. But the previous game had lengthy in-game descriptions of each freakish beast. Does knowing any of the following have much effect on gameplay? Probably not.
What's more is that, at least in the case of the sequel, the monsters of mayhem have their own background stories. They also have to look outrageous, frightening, or goofy.
It's not enough for them to simply come in ridiculous droves. One of Serious Sam's defining characteristics is its over-the-top, larger-than-life adversaries.