I'd heard tell in the past that it's used when the social rules are changed in the new location (so you wouldn't "get up from the sofa and go to kitchen"), but I think I may have worked up a better notion. Let's say you're Commonwealthy* and heard that Bob has "gone to hospital" - do you imagine Bob has health problems, or do you imagine that Bob is visiting someone with health problems?
My current hypothesis is that you'd imagine the former and if Bob was just visiting his sick aunt, he would have "gone to the hospital"; one would drop the "the" if one is using the service provided by the named location instead of just going to the location (so Bob's aunt had to go to hospital, and when Bob heard about it, he had to go to the hospital. Or Bob went to pub, so Bob's wife had to go to the pub to drive him home).
Oh, and on an unrelated note, I'd meant to post about a dream I had the other week.
In the dream-setting, it was quite standard for "crazy street-folks" to rant about unseen horrors and to collapse/scream in fits now and again. Something had happened to me (I can't recall exactly what it was) and I suddenly could see (a la Stephen King's "Insomnia") a new layer of reality that explained a bit of oddness.
Basically, there were Tim Burton style ghosties & ghoulies & long legg'd beasties stalking about, unseen by most, but plainly visible and solid to crazy folks. The ranting was just them pointing out the (to them) obvious, and the fits were brought about when the monsters would descend upon one of them (much like Abdul Al-Hazred meeting his marketplace demise by being torn apart by invisible demons).
I realized that I should probably pretend I couldn't see them, but then wasn't sure what to feign when it came to physical contact when some kappa-looking thing was walking close to me. Should I allow it to bump up against me and act like I didn't notice? Would the very fact that it could bump up against me be a sign, since they're normally intangible to most folks? How could I best observe the seemingly crazy folks and work out the physics at play here, without giving myself away?
*Do non-UK Commonwealthy folks drop the occasional "the" in this same way?