Cats evolved from solitary hunters, who used to spend the whole day looking for prey. And now, they spend most of their life sleeping on our couch and eating without much effort. Although it might sound like a good plan to us, our domestic cats still need to perform predatory behaviors, so they need to hunt even if they are well-fed and not hungry.
Although hunting indoors might seem difficult, using appropriate toys can be an alternative to keep them engaged, mentally stimulated and to facilitate predatory behavior. Avoiding this way, boringness and the subsequent problems associated that cats can develop such as overeating, over-grooming, separation anxiety, frustration, etc