This is what the trunk of Pixie the Prius looks like right now.

I have 2 yoga mats, a yoga towel, severable recyclable shopping bags, a 30lbs tub of kitty litter, a container with a picnic blanket, a baseball cap, a Dodgers blanky, a couple of makeshift dog leashes (for the occasional dog I end up rescuing off the street), a David Letterman sweatshirt, an umbrella (which I use maybe less often than the makeshift leashes), and a couple of "it fits, it ships" US Post Office boxes. Often my tripod will be in there, too.
As an aside, I almost tidied up before taking this picture. I thought if I stuffed the bags inside each other and folded them properly, it would make the trunk much tidier looking. But having decided to reveal my mini hoarding corner, I cleaned up after I took the picture instead.
I used to look at people's packed trunks and think, really, it's not that hard to keep your trunk clean. Well, it turns out I was right, it's not that hard. But the point is not whether it's easy, the point is, is there any point to it? The inside of my car is pretty clean. I have a Thomas Guide in there (really, I do, though I can't think the last time I used it), and maybe a roll of toilet paper (you never know), but other than that it's passenger-ready.
The trunk on the other hand seems like the perfect mobile mini-storage space to me. Why take the yoga mat out of there when I have to bring it with me every single time I go to yoga, which happens to be several times a week? The box with the picnic blankets and various and sundry bits is in there precisely for those times I go somewhere picnic-y and I forget a towel to sit on, my hat, or what have you. The leashes - well, that's self-explanatory. The kitty litter is temporary - it's just that it's heavy and I always wait till the last possible minute to bring it upstairs because I hate to carry the thing. The recyclable bags: again, why take them out of the car, when I only ever use them when I go shopping in my car? And so on.
You might say: if you were a better woman, and perfectly organized as good women should be, you would have a place for everything and everything in its place, and you would take your yoga mat from your closet each and every time you go to yoga class and return it to the closet when you get home. And you would not forget your hat or your picnic blanket or your umbrella when you might need it.
Alas, I fall short of the good woman in more ways than I can count, so I ask myself, why should I keep the trunk of my car always perfectly empty and clear? And I can't think of a single good reason.