Okay, let's stop with the nonsense. The reason some people bedline their trucks is as follows:
RUST is a major issue for some of the older trucks. If you have a rig that you beat on regularly, or just have a rig with paint that has seen better days, you come to a point where the truck needs to be painted to keep the body panels from rotting off. It doesn't matter if you have a trail-only build or run it on the street. Eventually it is going to need paint.
Because some of these guys beat the shit out of their trucks on the weekends, they don't want to pay some body shop to paint them only to need touch ups after a week. Therefore many people rattle can their paint with Rustoleum. When you scratch it up, you just touch it up as needed. Rustoleum even makes a clear coat, and if you know what you're doing and are willing to do the work, you can wet sand it to a finish that many people wouldn't know isn't automotive paint.
But since we're now in the realm of do-it-yourself paint jobs, some people have surmised that they could have a more durable finish that doesn't need to be touched up as often. Enter the bedliner. It's more durable and will likely protect the body from rust longer. A lot of guys bedline their lower quarters and rocker panels because of the extra abuse that those areas receive. It makes sense that someone would extend that to the whole vehicle.
Yeah, you have some mall crawlers do it just for the look. But some people actually do have a good reason for doing it.
By the way, before you shit on the idea too much, take a look at this picture:
Notice the black rocker panels? That's not regular paint. When Triumph painted these cars at the factory, they coated the rockers with a thick black layer very similar to bedliner, back before bedliner was even a thing. That's a far cry from bedlining an entire car, but the stuff does have legitimate uses even on sports cars.