Unless one is on a multi week expedition style, (not camping in the backcountry of VA), I don't get it.
I think you did get it for many people who have off road trailers, "multi week expedition style". To that I'll add the other group, big families that fill up a truck with people leaving little room for gear.
You always want to keep the vehicle centre of gravity as low as practical. A trailer is a way to take gear off the roof rack and the top layer or three out of the pile in the back of the truck.
Personally I am thinking about a trailer for a few reasons.
1. I still use film and I have a home film developer that fits into a large suitcase (ATL1000). I would love to be able to develop the day's sheets of film that evening. There's nothing like getting home and discovering you should have exposed the film a half stop more or that you missed a piece of crud on the lens. I think it would be fun to set up a film developing lab in an off road trailer.
2. My truck only holds 42 gallons of fuel in its tanks and my built in water tank is 15 gallons. I have to come into a town about once a week for fuel, water and food. A trailer would allow me to carry some fuel and water cans and a second refrigerator for use as a freezer (How cool would a helping of Rocky Road be when you have been out in the desert for a week). A trailer would allow me to stay out longer than a week without having to make a trip in just to restock and go back out.
3. My Truck is a Dormobile so the rear interior of my truck likes to stay clean and unscratched. a trailer will allow me to carry the stuff you normally toss into the back of a pickup around home.
I gather from your postings that you are a minimalist camper. I used to camp that way when I was in my teens camping with my Dad on father daughter fishing trips, and into my twenties when I was just taking my dog to see what was over the next hill. But those were weekend or long weekend trips and when the trip was done I was always happy to get home.
One thing I discovered about myself over time is that I can put up with just about anything for a short period but the more comfortable I am, the longer I want to stay out on a trip. To me a successful trip is when you are out and the desire to see what's around the next bend is stronger than your desire for the comforts of civilization. When a motel room and restaurant seem more appealing than another night on the trail its time to call it a trip.
My truck is set up to be as comfy and home like as possible. Comefy enough that I consider a month trip to be a normal trip length and come in because of obligations rather than because I miss the comforts of home. So far the longest I've been out is 3 months and I finally came in because I got tired of sub freezing mornings. A small trailer would allow me to stay out longer, take the gear off the roof rack and allow me to carry gear for different seasons.
Its all a matter of perspective. I think trailer would be a good idea for some but nowhere near all the places I travel. But I consider a week long outing to be a short trip, a month long trip to be normal and two months & longer to be the special trips. To me a weekender with a trailer would only be worth the effort if I'm bringing along spouse plus a gaggle of kids.