Quincy’s answer:

“Because we can!" Seriously, folks, what’s a feline to do? We have all sorts of important info to transmit—this is my territory; I’m a mature male; I passed by on Wednesday night. We can communicate who, where, when, and even why with the chemical substances called pheremones in our urine. Can humans do that? Suppose some fellow’s trying to invade my turf. I can warn him off without having to raise a paw. I’m really a gentle guy—neutered and everything... Just think how much more I’d have to spray about—and how much stronger it would smell—if I still had all my parts! (My friend Bettye performed The-Procedure-We-Do-Not-Name when I was about six months old, but I don’t hold that against her.) Apparently neutering controls about 90% of the marking problem in cats.

Humans have computer dating, but technology can’t replace pheremones, so when a female cat of breeding age and inclination sprays, the hormones in her urine stimulate certain responses in the intact males of the species. Those hormones can transmit for miles—it’s not cyber-space, but it works.
Dogs mark their territories, too, but they lack subtlety. They just let go over the same spot as the canines who preceded them, while we felines prefer to keep our scents distinct—that’s why we spray nearby.
But why indoors, you ask? Why the new couch? That determined backstep, the quivering tail, followed by a fragrant spray of urine lets interlopers know who’s in charge. The couch is mine—no doubt about it. Sometimes we’ll spray somewhere that our human companion (I hate that word master) frequents so there’s no question about who belongs to who. That’s why a new cat in the household can bring about a flurry of marking from a resident feline who’s never shown the inclination. Even outdoor prowlers (including those unfortunate fellows called feral cats) can stimulate the need to mark our indoor territory—after all, they might sneak inside during an unguarded moment. You might try removing bird feeders (drat!) and even installing motion-activated sprinklers to discourage these outsiders. And I’ll let you in on a little-known secret—we cats hate orange peels and most everything citrus, so you can use those nasty products inside to discourage us. For some felines, scratching makes a good substitute for marking, and a nice scratching post can do the trick—but don’t replace it just when we’ve got it broken in. And keep those litter boxes CLEAN—you may not mind a dirty bathroom, but we do.
Please feel free to use one of those ammonia-based cleaning products—409 is a particular favorite of many of my fellow felines, since it actually encourages re-spraying! Unfortunately, the humans here at Morningstar sell an insidious liquid called Nature’s Miracle that actually neutralizes urine and its lovely odor. Oh well... Morningstar also carries an amazing pheremone product called Feliway that can transform spraying behavior to innocuous head-rubbing (which is another way we can transmit information.) Surfaces marked by pheremones give us a sense of familiarity and well-being-- and we cats definitely like making ourselves at home.
“ We can communicate who, where, when, and even why with the chemical substances called pheremones...”

Quincy’s Answer:

What’s so great about perfume? Sometimes clients come to the clinic wearing so much sickly-sweet scent I have to leave the room. Catnip, on the other hand— ahhh, the bouquet, the pungent delight! I’m not sure I should be telling you this, but catnip (also known as catmint) actually contains the hallucinogenic compound nepetalactone, which does a pretty good imitation of the scent of feline sex pheromones. Which explains why many of us (including we individuals who have undergone The Procedure-We-Do-Not-Name) get so excited. A catnip high (fresh is best) usually lasts from five to fifteen minutes, making us cavort and roll, acting as if we’ve temporarily lost our minds. Don’t worry—catnip isn’t addictive. About 80% of all felines—including my larger cousins like lions and tigers—are sensitive to its effects; the rest are immune, poor things.....
Catnip Cookies
Preheat oven to 350 degrees.

1 c. whole wheat flour
1/4 c. soy flour
1 tsp. catnip
1 egg
1/3 c. milk
2 tbl. wheat germ
1/3 c. powdered milk
1 tbl. unsulfured molasses
Mix dry ingredients together, then add molasses, egg, and milk.
Roll dough out flat on a greased cookie sheet and cut into small pieces.
bake for 20 minutes at 350 degrees.
Serve when cool and store in a tightly sealed container.