Just out of high school I worked for an appliance delivery, installation, and repair. On a installation of a refrigerator with ice maker, my boss the owner told me I needed to crawl under the house and install a saddle water valve. We always drilled them out clean when installing them. He told me to crawl back where there was a copper line sticking through the floor and there would be one copper line under the house, that would be the water valve. I could not even roll over it was so tight. I found the line sticking through the floor, found the only copper line anywhere close, and after I drilled it, I heard the hiss and smelled the gas. I yelled at the boss to shut it off. He laughed, I thought you shut the water off. I yelled, THE GAS. The damned water line was about 40 feet from there and we had to fix a gas line under the house to boot. The cordless arching drill made me nervous along with propane gas spewing under the house with me.
You are right, there are companies that will go under your home and install plastic to keep the moisture from seeping up through the dirt into your crawl space. This can increase the humidity to the point where you get mold etc. if you don't control it. It is also nice to be crawling on clean plastic IF you have to go under. I still hate going under and avoid it if at all possible. Another reason it is worth the money to get a basement when you build.