Cats grow sharp claws- if left, they can grow into their paw pads. (OUCH)
Cats are greasy, they shed, and are usually sleeping in areas of the house that we clean the LEAST, ie under our beds, up on our refrigerator, and on top of cupboards.
Cats can pick up fleas, develop dandruff, and suffer from hairballs. (Who likes to clean up hairballs?) Their fur can also become matted.
* These are just a few of the reasons why cat owners seek out grooming.