What is the reason for that?
It was once popular to slip a piece of fuel line over the exposed ends of the chain adjuster, especially on bikes that were used off-road to protect the threads from dirt. That way if you had to make a quick chain adjustment by the side of the trail it was less likely that the adjuster nut would seize on the adjuster and possibly strip the thread. At least that's why I use to do it on my Enduro bikes. On street bikes it keeps the threads clean and gives the adjuster a finished look.
6504me can better explain why he did it, but I know we have similar backgrounds so I think his thinking would be similar.