You really don't need to. They are self-maintenance items. If there is damage to one (similar to a chipped nail on humans), then perhaps you should give it a trim - but with a specialized claw trimmer and not your nail clipper.
Ugh, you have too!!!!!!!!!!!!! Otherwise they will get an ingrown nail, then they won't be able to sheathe it, then, since it's curved, it will eventually start ripping their paw paw and toe. It's very painful to cats if you don't, and it sometimes even causes them to limp. Then you will have to spend even more money to get it fixed at the vet. :T Just cut it!!!! But make sure you don't cut the follicle.
Because they can claw things. But personally, I wouldn't. I had a cat when I was younger and she would rip our window screens, so my mom clipped her nails...sadly, her nails were the only way of defending herself, so she got eaten/attacked by dogs...
When I was a vet tech, we would have cats come in who's owners had not kept up at all with their claws. Often they were coming in for something unrelated, the owner would not even have noticed, and we would see it and have to clip them. After a certain point, they curve and grow INTO the pad of the foot. We would have to clip it, then actually pull it out of their pad. They usually bled.