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The most common use of these in my direct, personal, experience is that they were used to mark road construction / obstructions at night, before battery powered flashing lights replaced them. 

It's basically a crude lantern, typically fueled with diesel fuel. The wick is inside the structure on the top, which helps to keep it from blowing out completely in the wind, and keeps direct rain off the wick.

I'm sure it could also be used as @cobaltduck says, but I saw hundreds of them used on roadworks and I've never seen one in crops (but most of the crops around where I grew up were not particularly frost sensitive.)