Since devil grass--that addictive weed that gives off a greasy, smoky light when used to make a fire--was all there was to burn, both Roland and the Man in Black used it to start their campfires in the desert.
The Man in Black's campfires were set up in a definite pattern, one described as "ideographic" (clearly symbolic). Even the unimaginative Roland ponders the remains of the Man in Black's fires, whether for example they are spelling out a warning such as "Keep your distance, partner", "The end draweth nigh", or "Come and get me."
Roland's own campfires send a rather telling message about their creator. Described as a "straightforward crisscross" that is "vaguely frightening in its no-nonsense surety", Roland's method of setting up the devil grass to burn is a strong indicator of his simple, workmanlike approach to life.
Although border dwellers were frightened of devil grass fires, perhaps with good reason (there was an overwhelming belief that demons danced within the flames and would draw you in if you looked too closely ... or too long ... or at all), neither the gunslinger nor the Man in Black had any choice in what to make his fire with.
To me, the most telling message sent forth to the reader by contemplating the devil grass campfires is that, when coming upon the remains of one abandoned by the Man in Black, Roland eats a piece of charred bacon left behind amidst the pattern of symbols.
This tells me two things:
First, the Man in Black has no intention of harming the gunslinger ... and they both know this (or else the Man in Black might have poisoned the bacon ... or Roland would never have eaten it).
Second, it's possible (even probable) that the Man in Black is taking steps to keep Roland alive. Throwing the gunslinger scraps like a starving dog must give him some sort of perverse pleasure.