If you hang with cats, you've seen this pix a kzillion times. Cats snooze up to 18 hours a day, whether they need it or not. Especially in a warm patch of sunlight. A cat can't walk through that patch without a fatal attack of the sneeps.
Ok, so cats sleep a lot. What does that have to do with the purpose of cats, you ask. It's easy. If you're sensitive to the catzone, you can easily see the little balloon over this cat's head, can't you. Right. And in that little balloon are lots and lots of zzzzzzzzzzs. You've got it! Cats produce zzzzzzzzs. Billions, trillions, quadrattaillions of zzzzzzzzzzzzzzzzzzzzzzzzss. They good-naturedly, selflessly produce scads of excess zzzzzzzzzzzzzssssssssss just so I can catch the extra wink or two when I need it. Or for that matter, when you or anyone else needs it.
For example, I woke up too early today. About 4 am. I got up as I always do since I hate "trying" to sleep. I went back to be later, and thanks to my cats' production of excess zzzzzzzzzzzzzzzzzzzs, I was able to drop back off in Morpheus' arms and catch another 40 winks.
From now on, whenever you see cats snoozing, napping, lazing about with their eyes closed, smile at them and thank them. With maybe a dose of melatonin, a cat's excess zzzzzzzzs are all you need for a good night's sleep. Hell, my two cats even make enough to let the Gruff Lord snooze in naps daily!