The potential for character growth. The male reader can start as an ordinary person and through his encounters with the predator, transform. He might learn new survival skills, gain a deeper understanding of the universe, or even develop a new sense of self - worth. The predator, too, can be shown in a different light, perhaps not just as a mindless killer but as a complex being with its own motives and emotions. This exploration of two very different characters is what makes these fanfics so interesting.
The element of danger. Predators are known for being fierce hunters. In a 'predator x male reader fanfic', this danger factor is always present. It could be that the male reader is being hunted, or they might end up in a situation where they have to work together despite the predator's natural inclination to hunt. This constant threat or uneasy alliance makes for an exciting read. It's like a dance between life and death, and seeing how the male reader navigates this is very engaging.