Well it's a space to crawl in. When Mom and Dad aren't home the kids can play all kinds of spooky games in there. When Mom and Dad are home late at night its so the kids in their beds can hear a noise in there and wonder what kind of scary thing is sneaking around. The real purpose is to allow air circulation and access to plumbing, electrical, and other such for maintenance, repair, and installation. Homes without basements and not on a slab have a crawlspace underneath, homes without a real attic have a crawl space above the ceiling and below the roof. Some buildings, especially commercial types, have a crawl space between some walls. Older homes in areas where basements are uncommon had nice tall foundations with high crawl spaces that allowed cooling air to circulate under there, some of those are real easy to move around in, allmost walk spaces or at least stoop over spaces. I have spent considerable time in various crawl spaces dragging wires, pipes, hoses lumber, lights, and tools and can tell you some of them are actually squirm spaces, even nice roomy crawl spaces are head banging get into an awkward position with no room to swing a hammer put your arm behind your neck to turn a wrench spaces, and I have been in some that are real nice getting stuck spaces. Spiders, snakes, rats, possums, and skunks like to live in crawl spaces, especially cramped dark ones that are hard to get out of in a hurry. Some people can move backwards faster than forwards, takes 15 minutes to struggle in to to work site then a yelp or even a screech is heard and they come shooting out feet first in 30 seconds gasping, "Sn sn bi bi snake! Big snake!"