In the past 3ish years or so , it seems most people do this mod for what Keb88 wrote - for the space, added fuel capacity or both.
People also do it when they customize the truck, all different interior, high $ radio, etc.. It seems a lot of people take it out when they put in bucket seats, not everyone but I have noticed that trend.
Also for the space to stash stuff.
There is a good tech article on jim carter's site about why the tank is in the cab anyway and and good reasons for leaving it there.
My attitude is if the in tank cab punctures in a crash - the crash was pretty dang severe and yer prolly dead.
As long as ya keep the seals on the tank in good shape, ya should be good. It is my understanding gas needs to be a vapor to combust right, hence why carburetors and fuel injectors do what they do how they do it.
Mostly I've seen people take it out to add space in the cab, custom stuff, or they just wanted a bigger tank.
-W
