EDIT: /u/extesser is right, it's mostly dominated by heat conduction and not a temperature gradient. I'll leave the post untouched for the history books, since it sparked some interesting replies!

I can only speak for the Vikings in Norway. Temperature is one reason for why it emerges. Cold air sinks, hot air rises.

If you're a Viking living in a longhouse(ref: http://www.hurstwic.org/history/articles/daily_living/text/longhouse.htm ), sleeping on the floor might just kill you. Get up on a bench, and you're much better off.

Ideally you'd want to stay up against the roof if you wanted to stay as warm as possible - but smoke from the firepits collect there so the practical middleground would be benches or elaborate furniture with legs.

I can't speak about what they do in the tropics, but I know from personal experience that bugs alone make elevated beds an attractive solution.

I can speak for Middle Ages Nobility; Beds were raised about a foot off the ground to deter mice from nesting under them. If a bed was too low to the ground, the small crack could attract mice, rats and other creatures for use as a nesting spot.

i can only speak for Bedouin culture, but sleeping on mats, rugs, and furs is common until today, necessitated by the fact that they live a nomadic lifestyle, so rolling up your bed and throwing it on the back of your pack-animal is infinitely more convenient and practical than, say, disassembling and lugging around a four-post bed. Also, the air is slightly cooler on the ground, and that's where the drafts are felt.